<PAGE>
                                                                  Exhibit 10.120


                            CONTRIBUTION AGREEMENT

      THIS CONTRIBUTION AGREEMENT (the "Agreement") is made as of the 11th day
of December, 1997 (the "Effective Date") by and among CORPORATE 500-PHASE I, an
Illinois limited partnership ("Phase I Owner"), CORPORATE 500, PHASE II, an
Illinois limited partnership ("Phase II Owner"; Phase I Owner and Phase II Owner
are sometimes together called "Contributors" and individually a "Contributor"),
and CORNERSTONE PROPERTIES, INC., a Nevada corporation ("Cornerstone").

                                   RECITALS

      A. Phase I Owner is the beneficial owner of the Phase I Property (as
hereinafter defined) and Phase II Owner is the beneficial owner of the Phase II
Property (as hereinafter defined).

      B. Prior to Closing (as hereinafter defined), Cornerstone will form
Cornerstone Properties, L.P., a Delaware limited partnership (the "Operating
Partnership") having Cornerstoneas its general partner, and will contribute all
of its properties to the Operating Partnership.

      C . Contributors desire to contribute the Property (as hereinafter
defined) to the Operating Partnership in exchange for limited partner units in
the Operating Partnership ("Units"), each Unit being convertible into one share
of common stock of Cornerstone.

      NOW, THEREFORE, in consideration of and in reliance upon the above
Recitals, the terms, covenants and conditions contained herein, and other good
and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereby agree as follows:

                                  ARTICLE I

                    CONTRIBUTIONS TO OPERATING PARTNERSHIP

      1.1 Contribution of Phase I Property. Subject to the terms and conditions
hereinafter set forth, Phase I Owner agrees to contribute the following
described property (the "Phase I Property") to the Operating Partnership:
<PAGE>

      (a) (i) that certain tract or parcel of land situated in Lake County,
Illinois, more particularly described on Exhibit A-1 attached hereto and made a
part hereof, and (ii) all of Phase I Owner's right, title and interest in and to
that certain tract or parcel of land situated in Lake County, Illinois, more
particularly described on Exhibit A-2 (the "County Parcel") as provided in that
certain Agreement dated July 7, 1994, as amended, with the County of Cook
together with all and singular the easements, privileges and other rights and
appurtenances pertaining to such property, including any right, title and
interest of Phase I Owner in and to adjacent streets, alleys or rights-of-way
(the property described in clause (a) of this Section 1.1 being herein referred
to collectively as the "Phase I Land");

      (b) the buildings, structures, fixtures and other improvements on the
Phase I Land, including specifically, without limitation, the two office
buildings located thereon having a street address of 500 and 510 Lake Cook Road,
Deerfield, Illinois (the property described in clause (b) of this Section 1.1
being herein referred to collectively as the "Phase I Improvements");

      (c) all of Phase I Owner's right, title and interest in and to all
tangible personal property upon the Phase I Land or within the Phase I
Improvements, including specifically, without limitation, the items listed on
Exhibit B-1 attached hereto and made a part hereof and all other appliances,
furniture, carpeting, draperies and curtains, tools and supplies, all restaurant
equipment, fixtures and furniture used in connection with the operation of the
restaurant facilities in the Phase I Improvements, and other items of personal
property (excluding cash or escrow or other deposit accounts) used exclusively
in connection with the operation of the Land or the Improvements (both as
hereinafter defined) (the property described in clause (c) of this Section 1. 1
being herein referred to collectively as the "Phase I Personal Property");

      (d) all of Phase I Owner's right, title and interest in and to all
agreements listed and described on Exhibit C-1 (the "Phase I Lease Schedule")
attached hereto and made a part hereof, and all leases and agreements entered
into subsequent to the date hereof in accordance with the terms of this
Agreement pursuant to which any portion of the Phase I Land or Phase I
Improvements is used or occupied by anyone other than Phase I Owner (the
property described in clause (d) of this Section 1.1 being herein referred to
collectively as the "Phase I Leases"); and

      (e) all of Phase I Owner's right, title and interest in and to (i) all
contracts and agreements (collectively, the "Operating Agreements") listed and
described on Exhibit D (the "Operating Agreements Schedule") attached hereto and
made a part hereof, relating to the upkeep, repair, maintenance or operation of
the Land, Improvements or Personal Property (as hereinafter defined) which will
extend beyond the date of Closing (as such term is defined in Section 5.1
hereof), including specifically, without limitation, all assignable equipment
leases, (ii) all assignable existing warranties and guaranties (expressed or
implied) in favor of Phase I Owner in connection with the Phase I Improvements
or the Phase I Personal Property, and (iii) all assignable licenses and other
assignable governmental permits relating to the Phase I Land and Phase I
Improvements (the property described in this Section 1.1 (e) being sometimes
herein referred to collectively as the "Phase I Intangibles").


                                       2
<PAGE>

      1.2 Contribution of Phase II Property. Subject to the terms and conditions
hereinafter set forth, Phase II Owner agrees to contribute the following
described property (the "Phase II Property") to the Operating Partnership:

            (a) that certain tract or parcel of land situated in Lake County,
      Illinois, more particularly described on Exhibit A-3 attached hereto and
      made a part hereof, together with all and singular the easements,
      privileges and other rights and appurtenances pertaining to such property,
      including any right, title and interest of Phase II Owner in and to
      adjacent streets, alleys or rights-of-way (the property described in
      clause (a) of this Section 1.2 being herein referred to collectively as
      the "Phase II Land");

            (b) the buildings, structures, fixtures and other improvements on
      the Phase II Land, including specifically, without limitation, the two
      office buildings located thereon having a street address of 520 and 540
      Lake Cook Road, Deerfield, Illinois (the property described in clause (b)
      of this Section 1.2 being herein referred to collectively as the
      "Improvements");

            (c) all of Phase II Owner's right, title and interest in and to all
      tangible personal property upon the Phase II Land or within the Phase II
      Improvements, including specifically, without limitation, the items listed
      on Exhibit B-2 attached hereto and made a part hereof and all other
      appliances, furniture, carpeting, draperies and curtains, tools and
      supplies, all restaurant equipment, fixtures and furniture used in
      connection with the operation of the restaurant facilities in the Phase II
      Improvements and other items of personal property (excluding cash or
      escrow or other deposit accounts) used exclusively in connection with the
      operation of the Land or the Improvements (the property described in
      clause (c) of this Section 1.2 being herein referred to collectively as
      the "Phase II Personal Property");

            (d) all of Phase II Owner's right, title and interest in and to all
      agreements listed and described on Exhibit C-2 (the "Phase II Lease
      Schedule") attached hereto and made a part hereof, and all leases and
      agreements entered into subsequent to the date hereof in accordance with
      the terms of this Agreement pursuant to which any portion of the Phase II
      Land or Phase II Improvements is used or occupied by anyone other than
      Phase II Owner (the property described in clause (d) of this Section 1.2
      being herein referred to collectively as the "Phase II Leases"); and

            (e) all of Phase II Owner's right, title and interest in and to (i)
      all Operating Agreements, (ii) all assignable existing warranties and
      guaranties (expressed or implied) in favor of Phase II Owner in connection
      with the Phase II Improvements or the Phase II Personal Property, and
      (iii) all assignable licenses and other assignable governmental permits
      relating to the Phase II Land and Phase II Improvements (the property
      described in this Section 1.2(e) being sometimes herein referred to
      collectively as the "Phase II Intangibles").


                                       3
<PAGE>

      The Phase I Land and the Phase II Land are herein sometimes together
called the "Land", the Phase I Improvements and the Phase II Improvements are
herein sometimes together called the "Improvements," the Phase I Personal
Property and the Phase II Personal Property are herein sometimes called the
"Personal Property," the Phase I Leases and the Phase II Leases are herein
sometimes together called the "Leases," the Phase I Intangibles and the Phase II
Intangibles are herein sometimes called the "Intangibles," and the Phase I
Property and the Phase II Property are herein sometimes together called the
"Property."

      1.3 Existing Loans; Operating Partnership Loan. The Property shall be
contributed to the Operating Partnership subject to the existing loans secured
by the Property (the "Existing Loans"). Cornerstone agrees to cause the
Operating Partnership to borrow at least $80,000,000 on a non-recourse basis
secured by the Property, the proceeds of which loan (the "Operating Partnership
Loan") will be used to repay a portion of the Existing Loans.

                                  ARTICLE II

                       CONSIDERATION FOR CONTRIBUTIONS

      2.1 Consideration to Contributors. In exchange for the contribution of the
Property, and upon execution and delivery of the Partnership Agreement by
Contributors, Contributors shall receive, at the Closing, a number of Units
(rounded to the nearest whole number) equal to (a) One Hundred Fifty Million
Dollars ($150,000,000) less (b) the sum of (i) all unpaid principal of and
accrued interest and other charges and amounts outstanding under the Existing
Loans and (ii) closing costs, net prorations, and adjustments charged against
Contributors pursuant to this Agreement, divided by (c) Eighteen and 50/100
Dollars ($18.50). Each Contributor shall be entitled to receive the number of
Units as set forth in a written notice from Contributors to Cornerstone given at
least one business day prior to the date of Closing.

      In consideration of the contribution of the Property by the Contributors
and subject to the terms and conditions of this Agreement, Cornerstone shall pay
at the Closing (A) all amounts payable under the Existing Loans and (B) those
closing costs charged against Contributors pursuant to clause (b)(ii) above and
shall cause the Operating Partnership to issue the Units in accordance with the
terms of this Agreement; provided, however, that in no event shall the aggregate
amount of such payments by Cornerstone plus the product of $18.50 multiplied by
the number of Units issued by the Operating Partnership (determined as set forth
in this Section 2.1) exceed $150,000,000.

      2.2 Distribution of Units. At the Closing, the Operating Partnership shall
issue the Units either to Contributors or, at the option of Contributors,
directly to Contributors' partners (or to partners of such partners) in
accordance with written instructions provided to the Operating Partnership by
Contributors (or Contributors' partners) setting forth the name and address of,
and the number of Units to be received by, each partner, provided that each such
partner (i) makes


                                       4
<PAGE>

each of the representations and warranties set forth in Section 6.10 hereof,
(ii) completes and delivers to Cornerstone an investor questionnaire furnished
by Cornerstone, and (iii) has executed and delivered the Partnership Amendment
(as hereinafter defined).

      2.3 Earnest Money. Within one (1) business day following the execution and
delivery of this Agreement, Cornerstone shall deposit with Chicago Title and
Trust Company (the "Escrow Agent"), the sum of Five Million Dollars ($5,000,000)
(the "Earnest Money") in good funds, either by certified bank or cashier's check
or by federal wire transfer. The Escrow Agent shall hold the Earnest Money in an
interest-bearing account in accordance with the terms and conditions of an
escrow agreement entered into among Contributors, Cornerstone and Escrow Agent
simultaneously with the execution of this Agreement. All interest accruing on
such sum shall become a part of the Earnest Money and shall be distributed as
Earnest Money in accordance with the terms of this Agreement. Upon Closing, the
Earnest Money shall be returned to Cornerstone.

                                 ARTICLE III

                               TITLE AND SURVEY

      3.1 Title Examination; Commitment for Title Insurance. Contributor shall
deliver to Cornerstone, an ALTA title insurance commitment (the "Title
Commitment") covering the Land and Improvements from Chicago Title Insurance
Company (the "Title Company"), showing all matters affecting title to the Land
and Improvements and binding the Title Company to issue at Closing an Owner's
Policy of Title Insurance in the amount of $150,000,000 and a Mortgage Policy of
Title Insurance in an amount equal to the principal amount of the Operating
Partnership Loan. Contributors shall instruct the Title Company to deliver to
Cornerstone, Contributors and the surveyor described in Section 3.2 below copies
of the Title Commitment and copies of all instruments referenced in Schedule B
thereof.

      3.2 Survey. Contributors shall deliver to Cornerstone and the Title
Company an ALTA survey of the Land and Improvements (the "Survey") prepared by a
surveyor licensed by the State of Illinois and certified to Contributors,
Cornerstone, the Operating Partnership, the lender under the Operating
Partnership Loan and the Title Company in a manner reasonably acceptable to each
such party and reflecting the total area of the Land and Improvements, the
location of all improvements, recorded easements and encroachments, if any,
located thereon and all building and set back lines and other matters of record
with respect thereto.

      3.3 Title Objections; Cure of Title Objections. Cornerstone shall have
until the later to occur of (a) the expiration of the Inspection Period and (b)
five (5) business days from Cornerstone's receipt of the Title Commitment,
instruments of record and the Survey (the "Title Date"), to notify Contributors,
in writing, of such objections as Cornerstone may have to anything contained in
the Title Commitment or the Survey other than (i) real property taxes, not yet
due and payable; (ii) local, state and federal laws, ordinances and regulations,
including, without


                                       5
<PAGE>

limitations, building and zoning laws, ordinances and regulations now or
hereafter in effect relating to the Property (provided nothing herein shall
prevent Cornerstone from objecting to any violations thereof prior to the
expiration of the Inspection Period); (iii) right of tenants under leases, (iv)
the liens of the Existing Loans and (v) acts of the Operating Partnership and
parties acting by or through the Operating Partnership all of which shall be
deemed "Permitted Exceptions" hereunder. In addition to the Permitted Exceptions
listed in clauses (i) through (v), any item contained in the Title Commitment or
any matter shown on the Survey to which Cornerstone does not object by the Title
Date or during the period described in Section 3.5 shall be deemed a Permitted
Exception. In the event Cornerstone shall notify Contributors of objections to
title or to matters shown on the Survey on or prior to the Title Date in
accordance with the foregoing, Contributors shall have the right, but not the
obligation, to cure such objections (which cure may be effected either by the
removal, satisfaction or cure of same or, with the consent of Cornerstone (which
consent shall not be unreasonably withheld), by causing the Title Company to
issue an endorsement insuring over loss from the existence of such defect).
Within ten (10) days after receipt of Cornerstone's notice of objections,
Contributors shall notify Cornerstone in writing whether Contributors elect to
attempt to so cure such objections. If Contributors elect to attempt to cure,
and provided that Cornerstone shall not have terminated this Agreement in
accordance with Section 4.2 hereof, Contributors shall have until the date of
Closing to attempt to remove, satisfy or cure the same and for this purpose
Contributors shall be entitled to a reasonable adjournment of the Closing if
additional time is required, but in no event shall the adjournment exceed sixty
(60) days after the date for Closing set forth in Section 5.1 hereof. If
Contributors elect not to cure any objections specified in Cornerstone's notice,
or if Contributors are unable to effect a cure prior to the Closing (or any date
to which the Closing has been adjourned in accordance with this Section 3.3),
Cornerstone shall have the following options: (i) to accept a conveyance of the
Property subject to the Permitted Exceptions, specifically including any matter
objected to by Cornerstone which Contributors are unwilling or unable to cure,
and without reduction in the consideration to be received by Contributors; or
(ii) to terminate this Agreement by sending written notice thereof to
Contributors, and upon delivery of such notice of termination, this Agreement
shall terminate and the Earnest Money shall be returned to Cornerstone, and
thereafter neither party hereto shall have any further rights, obligations or
liabilities hereunder except to the extent that any right, obligation or
liability set forth herein expressly survives termination of this Agreement. If
Contributors notify Cornerstone that Contributors do not intend to attempt to
cure any title objection; or if, having commenced attempts to cure any
objection, Contributors later notify Cornerstone that Contributors will be
unable to effect a cure thereof; Cornerstone shall, within five (5) business
days after such notice has been given, notify Contributors in writing whether
Purchaser shall elect to accept the conveyance under clause (i) or to terminate
this Agreement under clause (ii).

      3.4 Conveyance of Title. At Closing, Contributors shall convey and
transfer to the Operating Partnership good and marketable title to the Property,
subject only to the Permitted Exceptions, it being agreed that either fee title
or valid and subsisting easement rights to the County Parcel shall be conveyed
to the Operating Partnership depending on whether the Phase I Owner has acquired
the County Parcel prior to Closing.


                                       6
<PAGE>

      3.5 Pre-Closing "Gap" Title Defects. Whether or not Cornerstone shall have
furnished to Contributors any notice of title objections pursuant to the
foregoing provisions of this Agreement, Cornerstone may, at or prior to Closing,
notify Contributors in writing of any objections to title first raised by the
Title Company or the Surveyor between (a) the effective date of the Title
Commitment referred to above, and (b) the date on which the transaction
contemplated herein is scheduled to close. With respect to any objections to
title set forth in such notice, Contributors shall have the same option to cure
and Cornerstone shall have the same option to accept title subject to such
matters or to terminate this Agreement as those which apply to any notice of
objections made by Cornerstone in accordance with Section 3.3. If Contributors
elect to attempt to cure any such matters, the date for Closing shall be
automatically extended by a reasonable additional time to effect such a cure,
but in no event shall the extension exceed sixty (60) days after the date for
Closing set forth in Section 5.1 hereof.

                                  ARTICLE IV

                              INSPECTION PERIOD

      4.1 Right of Inspection. During the period which began on December 3, 1997
and will end at 5:00 p.m. (local time at the Property) on December 22, 1997
(hereinafter referred to as the "Inspection Period"), Cornerstone shall have the
right to make a physical inspection of the Property, to perform tests on the
Property and to examine at such place or places at the Property, in the offices
of the property manager or elsewhere as the same may be located, any operating
files maintained by Contributors or their property manager in connection with
the leasing, maintenance and/or management of the Property, including, without
limitation, the Leases, lease files, Operating Agreements, insurance policies,
bills, invoices, receipts and other general records relating to the income and
expenses of the Property, correspondence, surveys, plans and specifications,
warranties for services and materials provided to the Property, engineering
reports, environmental audits and similar materials, but excluding materials
unrelated to the leasing, maintenance, construction, development, operation
and/or management of the Property such as Contributors' internal memoranda and
income tax records. At Cornerstone's request, Contributors shall afford
Cornerstone the opportunity to discuss the Property and the operation thereof
with Contributors' employees and agents and, after the end of the Inspection
Period, with any tenant under any Lease, at such reasonable times as Cornerstone
may from time to time request provided that Contributors shall be afforded the
opportunity to accompany Cornerstone in connection with any discussion with a
tenant under a Lease. Cornerstone understands and agrees that any on-site
inspections or testing of the Property shall be conducted upon at least
twenty-four (24) hours' prior written notice to Contributors and in the presence
of Contributors or their representative. Any such inspections and testing shall
be performed by companies selected by Cornerstone and approved by Contributors,
which approval shall not be unreasonably withheld. Cornerstone agrees to repair
any damage to the Property and to indemnify Contributor against and hold
Contributor harmless from any claim for liabilities, costs, expenses (including
reasonable attorneys' fees actually incurred) damages or injuries arising out of
or resulting from the


                                       7
<PAGE>

inspection or testing of the Property by Cornerstone or its consultants or
agents, and notwithstanding anything to the contrary in this Agreement, such
obligation to repair and to indemnify and hold harmless Contributors shall
survive Closing or any termination of this Agreement. All inspections and
testing shall occur at reasonable times agreed upon by Contributors and
Cornerstone and shall be conducted so as not to interfere unreasonably with use
of the Property by Contributor or its tenants.

      4.2 Right of Termination. Contributors agree that in the event Cornerstone
determines (such determination to be made in Cornerstone's sole discretion) that
the Property is not suitable for its purposes for any reason, Cornerstone shall
have the right to terminate this Agreement by giving written notice thereof to
Contributors prior to the expiration of the Inspection Period. If Cornerstone
gives such notice of termination within the Inspection Period, this Agreement
shall terminate and the Earnest Money shall be returned to Cornerstone. Time is
of the essence with respect to the provisions of this Section 4.2. If
Cornerstone falls to give Contributors a notice of termination prior to the
expiration of the Inspection Period, Cornerstone shall no longer have any right
to terminate this Agreement under this Section 4.2 and (subject to the
provisions of Section 3.5 and the other conditions to Closing set forth in this
Agreement) shall be bound to proceed to Closing and consummate the transaction
contemplated hereby pursuant to the terms of this Agreement.

      4.3 Partnership Agreement; Registration Rights Agreement. (a) Promptly
after the Effective Date, Cornerstone shall deliver to Contributors drafts of
the limited partnership agreement of the Operating Partnership (the "Partnership
Agreement") and a registration rights agreement between Cornerstone and the
Contributors (the "Registration Rights Agreement"). The Partnership Agreement
shall be subject to Contributors' review and approval during the Inspection
Period and shall be attached to this Agreement as Exhibit F. The parties agree
to negotiate the terms of the Registration Rights Agreement in good faith during
the Inspection Period and upon agreement thereof to attach the Registration
Rights Agreement to this Agreement as Exhibit G. In the event the Partnership
Agreement or the Registration Rights Agreement is not finalized by the
expiration of the Inspection Period, this Agreement shall terminate and the
Earnest Money shall be returned to Cornerstone. Notwithstanding the foregoing,
Contributors shall have the right to extend the time period for completion of
the Partnership Agreement or Registration Rights Agreement up to fifteen (15)
additional days by giving written notice thereof to Cornerstone prior to the
expiration of the Inspection Period.

      (b) The parties agree that the Partnership Agreement will provide, among
other things, that each Unit is convertible into one share of common stock of
Cornerstone.

      (c) The parties agree that the Registration Rights Agreement will provide,
among other things, (i) that within 90 days of written request from any person
or persons who hold at least one-third (1/3) of the Units issued at Closing,
Cornerstone shall cause a shelf registration statement to become effective for
shares issued upon the conversion of the Units which will continue to be
effective for a period of four years after the effective date of such
registration


                                       8
<PAGE>

statement, (ii) for piggyback registration rights, (iii) for the right of any
person holding at least 10% of the Units issued at Closing to request, at any
time prior to the time a request for a shelf registration statement is made and
at any time after any shelf registration statement is no longer effective, that
Cornerstone cause a registration statement be filed which will remain effective
for a period of at least 180 days, provided that no more than one such request
will be made by any one person prior to the filing of the shelf registration and
no more than one such request will be made by any one person after the shelf
registration statement is no longer effective, and (iv) for black out periods of
no longer than 90 days in length and no more than two such periods in any twelve
(12) month period.

      4.4 Completion of Other Exhibits. With respect to any other Exhibits to
this Agreement which are not attached to this Agreement at execution hereof, the
parties agree to complete said Exhibits and attach them to this Agreement prior
to the expiration of the Inspection Period.

                                  ARTICLE V

                                   CLOSING

      5.1 Time and Place. The consummation of the transaction contemplated
hereby ("Closing") shall be held at the offices of Contributor's counsel, Katten
Muchin & Zavis, 525 West Monroe Street, Suite 1600, Chicago, Illinois, at 10:00
a.m. on a date mutually agreed to by Contributors and Cornerstone, but in no
event later than January 31, 1998. At Closing, Contributors and Cornerstone
shall perform the obligations set forth in, respectively, Section 5.2 and
Section 5.4, the performance of which obligations shall be concurrent
conditions.

      5.2 Contributors' Obligations at Closing. At Closing, Contributors shall:

            (a) deliver to Operating Partnership duly executed trustee deeds
      (the "Deeds") in recordable form, conveying the Land and Improvements,
      subject only to the Permitted Exceptions, in form reasonably acceptable to
      Contributors and Cornerstone;

            (b) deliver to Operating Partnership duly executed bills of sale
      conveying the Personal Property without warranty of title or use and
      without warranty, expressed or implied, as to merchantability and fitness
      for any purpose, in form reasonably acceptable to Contributors and
      Cornerstone;

            (c) assign to Operating Partnership, and Operating Partnership shall
      assume, the landlord/lessor interest in and to the Leases by duly executed
      assignment and assumption agreements, in form reasonably acceptable to
      Contributors and Cornerstone;

            (d) to the extent assignable, assign to Operating Partnership, and
      Operating


                                       9
<PAGE>

      Partnership shall assume, Contributors' interest in the Operating
      Agreements and the other Intangibles by duly executed assignment and
      assumption agreements, in form reasonably acceptable to Contributors and
      Cornerstone;

            (e) deliver to Operating Partnership such Tenant Estoppels (as
      defined in Section 6.4(b) hereof) as are in Contributors' possession;

            (f) join with Operating Partnership to execute a notice in form and
      content reasonably satisfactory to Cornerstone and Contributors which
      notice Operating Partnership shall send to each tenant under each of the
      Leases informing such tenant of the sale of the Property and of the
      assignment to Operating Partnership of Contributors' interest in, and
      obligations under, the Leases (including, if applicable any security
      deposits) and directing that all rent and other sums payable after the
      Closing under each such Lease shall be paid as set forth in the notice;

            (g) deliver to Operating Partnership a certificate, dated as of the
      date of Closing, stating that the representations and warranties of each
      Contributor contained in this Agreement are true and correct in all
      material respects as of the date of Closing (with appropriate
      modifications of those representations and warranties made in Section 6.1
      or Section 6.2 hereof to reflect any changes therein including without
      limitation any changes resulting from actions under Section 6.5 hereof) or
      identifying any representation or warranty which is not, or no longer is,
      true and correct and explaining the state of facts giving rise to the
      change. In no event shall a Contributor be liable to Cornerstone or
      Operating Partnership for, or be deemed to be in default hereunder by
      reason of, any breach of representation or warranty which results from any
      change that (i) occurs between the Effective Date and the date of Closing
      and (ii) is expressly permitted under the terms of this Agreement or is
      beyond the reasonable control of such Contributor to prevent; provided,
      however, that the occurrence of a change which is not permitted hereunder
      or is beyond the reasonable control of such Contributor to prevent shall,
      if materially adverse to Cornerstone or Operating Partnership, constitute
      the non-fulfillment of the condition set forth in Section 5.7(b); if,
      despite changes or other matters described in such certificate, the
      Closing occurs, such Contributor's representations and warranties set
      forth in this Agreement shall be deemed to have been modified by all
      statements made in such certificate;

            (h) deliver to Cornerstone such evidence as Cornerstone's counsel
      and/or the Title Company may reasonably require as to the authority of the
      person or persons executing documents on behalf of Contributors;

            (i) deliver to Operating Partnership affidavits duly executed by
      Contributors stating that each Contributor is not a "foreign person" as
      defined in the Federal Foreign Investment in Real Property Tax Act of 1980
      and the 1984 Tax Reform Act;


                                       10
<PAGE>

            (j) deliver to Operating Partnership the Leases, Operating
      Agreements and licenses and permits, if any, in the possession of
      Contributors or Contributors' agents, together with such leasing and
      property files and records which are material in connection with the
      continued operation, leasing and maintenance of the Property. Operating
      Partnership shall cooperate with each Contributor for a period of seven
      (7) years after Closing in case of such Contributor's need in response to
      any legal requirement, a tax audit, tax return preparation or litigation
      threatened or brought against such Contributor, by allowing such
      Contributor and its agents or representatives access, upon reasonable
      advance notice (which notice shall identify the nature of the information
      sought by such Contributor), at all reasonable times to examine and make
      copies of any and all instruments, files and records, which right shall
      survive the Closing;

            (k) deliver to Operating Partnership possession and occupancy of the
      Property, subject to the Permitted Exceptions;

            (1) deliver to Cornerstone counterparts of an amendment to the
      Partnership Agreement (the "Partnership Amendment") executed by
      Contributors admitting the Contributors as limited partners in the
      Operating Partnership, in form reasonably acceptable to Contributors and
      Cornerstone;

            (m) deliver to Cornerstone counterparts of the Registration Rights
      Agreement executed by Contributors;

            (n) deliver to the Title Company a customary title affidavit and
      such other instruments as may be required by the Title Company to issue
      the title policies to be delivered hereunder;

            (o) deliver to Cornerstone evidence of the termination of the
      existing management agreements and the lease with Matas Corporation;

            (p) deliver to Cornerstone a pay-off letter or other evidence
      reasonably acceptable to Cornerstone of the aggregate amounts payable
      under the Existing Loans; and

            (q) deliver such additional documents as shall be reasonably
      required to consummate the transaction contemplated by this Agreement.

      5.3 Intentionally Omitted.

      5.4 Cornerstone's Obligations at Closing. At Closing, Cornerstone shall or
shall cause Operating Partnership, as applicable, to:

            (a) join Contributors in the execution of the instruments described
      in Sections 5.2(c), 5.2(d), 5.2(f), 5.2(l) and 5.2(m) above;


                                       11
<PAGE>

            (b) deliver to Contributors the Units as required by Article II
      hereof;

            (c) deliver to Contributors certified copies of the Partnership
      Agreement and the Certificate of Limited Partnership of the Operating
      Partnership;

            (d) deliver to Contributors such evidence as Contributors' counsel
      and/or the Title Company may reasonably require as to the authority of the
      person or persons executing documents on behalf of Cornerstone and
      Operating Partnership; and

            (e) deliver such additional documents as shall be reasonably
      required to consummate the transaction contemplated by this Agreement.

      5.5 Credits and Prorations.

            (a) The following shall be apportioned with respect to the Property
      as of 12:01 a.m., on the day of Closing, as if Operating Partnership were
      vested with title to the Property during the entire day upon which Closing
      occurs:

                  (i) rents, if any, as and when collected (the term "rents" as
            used in this Agreement includes all payments due and payable by
            tenants under the Leases, other than payments under the Tenant
            Improvement Loans (as hereinafter defined));

                  (ii) general real estate taxes and assessments levied against
            the Property for the years 1997 and 1998;

                  (iii) payments under the Operating Agreements;

                  (iv) gas, electricity and other utility charges for which
            Contributors are liable, if any, such charges to be apportioned at
            Closing on the basis of the most recent meter reading occurring
            prior to Closing;

                  (v) any accrued and unpaid interest under the Tenant
            Improvement Loans; and

                  (vi) any other operating expenses or other items pertaining to
            the Property which are customarily prorated between a purchaser and
            a seller in the area in which the Property is located.

            (b) Notwithstanding anything contained in the foregoing provisions:

                  (i) At Closing, (A) Contributors shall, at Contributors'
            option, either deliver to Operating Partnership any security
            deposits actually held by Contributors


                                       12
<PAGE>

            pursuant to the Leases or credit to the account of Operating
            Partnership the amount of such security deposits (to the extent such
            security deposits are not applied against delinquent rents or
            otherwise as provided in the Leases), and (B) Operating Partnership
            shall credit to the account of Contributors all refundable cash or
            other deposits posted with utility companies serving the Property,
            or, at Contributors' option, Contributors shall be entitled to
            receive and retain such refundable cash and deposits.

                  (ii) General real estate taxes for the year 1997, payable in
            1998, and for the year 1998, payable in 1999, shall be prorated at
            Closing based upon the amount of such taxes reasonably estimated by
            Contributor for purposes of tenants' monthly tax payments under the
            Leases. Operating Partnership shall be responsible for the payment
            of all such taxes. Upon issuance of the actual tax bills, taxes
            shall be reprorated as follows: (y) in the event the actual taxes
            are higher than the estimate therefor, Operating Partnership shall
            look solely to the tenants under the Leases for their share of such
            excess and Contributors shall pay to the Operating Partnership the
            portion of such excess which is not payable by tenants under the
            Leases, and (z) in the event the actual taxes are less than the
            estimate therefor, the Operating Partnership shall refund to tenants
            the portion of such excess payable to the tenants and shall pay to
            Contributors their portion of the balance.

                  (iii) As to gas, electricity and other utility charges
            referred to in Section 5.5(a)(iv) above, Contributors may on notice
            to Cornerstone elect to pay one or more of all of said items accrued
            to the date hereinabove fixed for apportionment directly to the
            person or entity entitled thereto, and to the extent Contributors so
            elect, such item shall not be apportioned hereunder, and
            Contributor's obligation to pay such item directly in such case
            shall survive the Closing.

                  (iv) Operating Partnership shall be responsible for the
            payment of (A) all Tenant Inducement Costs (as hereinafter defined)
            and leasing commissions which become due and payable (whether before
            or after Closing) as a result of (1) the leases, renewals or
            expansions described on Exhibit H-1 hereto, (2) any renewals or
            expansions of existing Leases in accordance with the terms of such
            Leases occurring after the Effective Date, and (3) any new Leases
            entered into after the Effective Date which are approved or deemed
            approved by Cornerstone pursuant to Section 6.6(c) hereof. The
            Tenant Inducement Costs and leasing commissions payable by the
            Operating Partnership with respect to the leases, renewals or
            expansions described on Exhibit H-1, as estimated as of the
            Effective Date, are listed on said Exhibit H-1. If, as of the date
            of Closing, a Contributor shall have paid any Tenant Inducement
            Costs or leasing commissions for which Operating Partnership is
            responsible pursuant to the foregoing provisions, Operating
            Partnership shall reimburse such Contributor therefor at Closing.
            Contributors shall be responsible for the payment of all Tenant
            Inducement Costs and leasing commissions under the Leases listed on


                                       13
<PAGE>

            Exhibit H-2 hereto which are payable after Closing. The Operating
            Partnership shall receive a credit at Closing for any such amounts
            which are unpaid as of Closing. For purposes hereof, the term
            "Tenant Inducement Costs" shall mean reasonable attorneys' fees and
            costs incurred in connection with the preparation and negotiation of
            a new Lease or a renewal or expansion of an existing Lease and any
            out-of-pocket payments required under a Lease to be paid by the
            landlord thereunder to or for the benefit of the tenant thereunder
            which is in the nature of a tenant inducement, including
            specifically, without limitation, tenant improvement costs, space
            planning costs, construction management fees, lease buyout costs,
            and moving, design and refurbishment allowances. The term "Tenant
            Inducement Costs" shall not include loss of income resulting from
            any free rental period, it being agreed that Contributors shall bear
            the loss resulting from any free rental period until the date of
            Closing and that Cornerstone shall bear such loss from and after the
            date of Closing.

                  (v) Unpaid and delinquent rent collected by Contributors and
            Operating Partnership after the date of Closing shall be delivered
            as follows: (a) if Contributors collect any unpaid or delinquent
            rent for the Property, Contributors shall, within fifteen (15) days
            after the receipt thereof, deliver to Operating Partnership any such
            rent which Operating Partnership is entitled to hereunder relating
            to the date of Closing and any period thereafter, and (b) if
            Operating Partnership collects any unpaid or delinquent rent from
            the Property, Operating Partnership shall, within fifteen (15) days
            after the receipt thereof, deliver to Contributors any such rent
            which Contributors are entitled to hereunder relating to the period
            prior to the date of Closing. Contributors and Operating Partnership
            agree that all rent received by Contributors or Operating
            Partnership after the date of Closing shall be applied first to
            current rentals and then to delinquent rentals, if any, in inverse
            order of maturity. Operating Partnership will make a good faith
            effort after Closing to collect all rents in the usual course of its
            operation of the Property, but Operating Partnership will not be
            obligated to institute any lawsuit or other collection procedures to
            collect delinquent rents. In the event that there shall be any rents
            or other charges under any Leases which, although relating to a
            period prior to Closing, do not become due and payable until after
            Closing or are paid prior to Closing but are subject to adjustment
            after Closing (such as reimbursements for year end operating expense
            reimbursements and the like), then any rents or charges of such type
            received by Operating Partnership or its agents or Contributors or
            their agents subsequent to Closing shall, to the extent applicable
            to a period extending through the Closing, be prorated between
            Contributors and Operating Partnership as of Closing and
            Contributors' portion thereof shall be remitted promptly to
            Contributors by Operating Partnership.

                  (vi) The aggregate principal amount owed Contributors by the
            tenants under the Leases described on Exhibit I hereto (the "Tenant
            Improvement Loans")


                                       14
<PAGE>

            for tenant improvement costs previously funded by Contributors, the
            reimbursement of which is not included in base rent, shall be
            credited to Contributors at Closing. The principal amounts
            outstanding under the Tenant Improvement Loans as of the Effective
            Date are set forth on said Exhibit I.

            (c) The provisions of this Section 5.5 shall survive Closing.

      5.6 Closing Costs. Contributors shall pay (a) the fees of any counsel
representing it in connection with this transaction, (b) the fee for the title
examination and the Title Commitment and the premium for the Owner's Policy of
Title Insurance to be issued to Operating Partnership by the Title Company at
Closing, (c) the cost of the Survey, (d) the transfer tax imposed by the State
of Illinois, the County of Lake and the City of Deerfield (if any), and (e)
one-half (1/2) of any escrow fee which may be charged by the Escrow Agent or
Title Company. Cornerstone shall pay (w) the fees of any counsel representing
Cornerstone or Operating Partnership in connection with this transaction; (x)
fees for recording the deeds conveying the Property to Operating Partnership;
(y) the fee for any endorsements required by Cornerstone to the Owner's Policy
of Title Insurance to be issued to Operating Partnership by the Title Company at
Closing; and (z) one-half (1/2) of any escrow fees charged by the Escrow Agent
or Title Company. All other costs and expenses incident to this transaction and
the closing thereof shall be paid by the party incurring same.

      5.7 Conditions Precedent to Obligation of Cornerstone. The obligation of
Cornerstone to consummate the transaction hereunder shall be subject to the
fulfillment on or before the date of Closing of all of the following conditions,
any or all of which may be waived by Cornerstone in its sole discretion:

            (a) Contributors shall have delivered to Cornerstone or Operating
      Partnership all of the items required to be delivered to Cornerstone or
      Operating Partnership pursuant to the terms of this Agreement, including
      but not limited to, those provided for in Section 5.2.

            (b) All of the representations and warranties of Contributors
      contained in this Agreement shall be true and correct in all material'
      respects as of the date of Closing (with appropriate modifications
      permitted under this Agreement).

            (c) Contributors shall have performed and observed, in all material
      respects, all covenants and agreements of this Agreement to be performed
      and observed by Contributors as of the date of Closing.

            (d) Cornerstone shall have received a Tenant Estoppel from Jim Beam
      Brands Company, Gaylord Container and MMI Companies, Inc. (the "Major
      Tenants") and a Tenant Estoppel or Seller Estoppel (as hereinafter
      defined) from other tenants of the Improvements such that Cornerstone
      shall have received Tenant Estoppels or Seller Estoppels from tenants
      occupying at least 70% of the rentable area of the Improvements which is
      leased and occupied as of the Effective Date which reflect matters which
      are not


                                       15
<PAGE>

      materially inconsistent with the representations and warranties of
      Contributors contained in Sections 6.1 (c), 6.1 (m), 6.2(c) and 6.2(m) of
      this Agreement.

In the event any of the foregoing conditions are not fulfilled or waived by
Cornerstone by Closing, this Agreement shall terminate and the Earnest Money
shall be returned to Cornerstone.

      5.8 Conditions Precedent to Obligation of Contributors. The obligation of
Contributors to consummate the transaction hereunder shall be subject to the
fulfillment on or before the date of Closing of all of the following conditions,
any or all of which may be waived by Contributors in their sole discretion:

            (a) Contributors shall have received the Units in the number
      provided for in this Agreement.

            (b) Cornerstone and Operating Partnership shall have delivered to
      Contributors all of the items required to be delivered to Contributors
      pursuant to the terms of this Agreement, including but not limited to,
      those provided for in Section 5.4.

            (c) All of the representations and warranties of Cornerstone
      contained in this Agreement shall be true and correct in all material
      respects as of the date of Closing.

            (d) Cornerstone shall have performed and observed, in all material
      respects, all covenants and agreements of this Agreement to be performed
      and observed by Cornerstone as of the date of Closing.

In the event any of the foregoing conditions are not fulfilled or waived by
Contributors by Closing, this Agreement shall terminate and the Earnest Money
shall be returned to Cornerstone.

                                   ARTICLE VI

                   REPRESENTATIONS, WARRANTIES AND COVENANTS

      6.1 Representations and Warranties of Phase I Owner. Phase I Owner hereby
makes the following representations and warranties to Cornerstone as of the
Effective Date:

            (a) Organization; Authority; Binding Obligations; No Violation.
      Phase I Owner has been duly organized and is validly existing under the
      laws of the State of Illinois. Phase I Owner has full right, authority,
      power and capacity: (i) to enter into this Agreement and each agreement,
      document and instrument to be executed and delivered by Phase I Owner
      pursuant to this Agreement; and (ii) to carry out the transactions
      contemplated hereby and thereby. This Agreement and each agreement,
      document and instrument executed and delivered by Phase I Owner pursuant
      to this Agreement constitutes, or when


                                       16
<PAGE>

      executed and delivered will constitute, the legal, valid and binding
      obligation of Phase I Owner enforceable against Phase I Owner in
      accordance with their respective terms. The execution, delivery and
      performance of this Agreement and each agreement, document and instrument
      executed and delivered by Phase I Owner: (x) does not and will not violate
      Phase I Owner's organizational documents; (y) does not and will not
      violate any federal, state, local or other laws applicable to Phase I
      Owner or require Phase I Owner to obtain any approval, consent or waiver
      of, or make any filing with, any person or authority (governmental or
      otherwise) that has not been obtained or made or which does not remain in
      effect; and (z) does not and will not violate or conflict with, or result
      in a breach of, or constitute a default under, any notice, mortgage,
      contract or agreement to which Phase I Owner is a party, or by which Phase
      I Owner or any of its properties is bound, other than the documents
      evidencing and securing the Existing Loans.

            (b) Pending Actions. To Phase I Owner's knowledge, except as
      described on Exhibit J-1 hereto, there is no action, suit, arbitration,
      unsatisfied order or judgment, governmental investigation or proceeding
      pending against the Phase I Property or the transaction contemplated by
      this Agreement.

            (c) Phase I Leases. The Phase I Leases listed in the Phase I Lease
      Schedule constitute all of the leases, licenses and occupancy agreements
      affecting the Phase I Property to which the Phase I Owner is a party
      (either directly or by successor to ownership of the Building) and, except
      as set forth on the Phase I Lease Schedule or on Exhibit K- 1 attached
      hereto: (i) the Phase I Leases have not been modified or amended, (ii) as
      of the date hereof, to Phase I Owner's knowledge, the Leases are in full
      force and effect, and the Phase I Owner has not sent written notice to any
      tenant under a Lease claiming that the tenant is in default under its
      Lease, which default remains uncured, and the Phase I Owner has no
      knowledge of any such default, and (iii) Phase I Owner has delivered to
      Cornerstone, true and complete copies of the Phase I Leases. Exhibit L-1
      attached hereto sets forth a true and correct schedule of all security
      deposits held by Phase I Owner pursuant to the Phase I Leases, specifying
      the tenant which deposited such security deposit. No interest is payable
      by Phase I Owner to any tenant with respect to such security deposits. To
      Phase I Owner's actual knowledge, there are no subleases under, or
      assignment of, the Phase I Leases except as set forth on the Phase I Lease
      Schedule. Except as set forth in the Phase I Leases, Phase I Owner has not
      granted to any person any right to acquire an interest in or to lease, or
      any right of first offer or first refusal with respect to, all or any
      portion of the Phase I Property. Except as set forth on Exhibit K-1, (A)
      each of the tenants under the Phase I Leases has taken possession of its
      respective demised premises and has commenced payment of rent, (B) all
      work allowances or other sums required to be paid as of the date hereof by
      the landlord under the Phase I Leases, and all work required to be
      performed by the landlord with respect to the preparation of such demised
      premises or the Phase I Property for the tenant's occupancy under the
      Phase I Leases, and any other work or other obligation required to be
      performed by the landlord under the Leases, and any other work or other
      obligation required to be performed by the landlord under the Leases, has
      been performed


                                       17
<PAGE>

      and/or paid for, (C) Phase I Owner has not received from any of the
      tenants under the Phase I Leases any written notice (i) claiming any
      default by the landlord under its Lease that has not been remedied, (ii)
      claiming that it is entitled to any offset, setoff or deduction against
      rent, (iii) disputing any rent computation that has not been resolved,
      (iv) advising Phase I Owner that it intends to vacate its premises upon
      the expiration of the term of its Phase I Lease or exercise any right to
      terminate its Phase I Lease, and (D) to Phase I Owner's knowledge, Phase I
      Owner is not in default of any of its obligations under the Phase I
      Leases, no tenant under the Phase I Leases is entitled to any offset,
      setoff, or deduction against rent and there are no disputes regarding rent
      computations that have not been resolved. In the event that any Tenant
      Estoppel delivered to Cornerstone with respect to any Phase I Lease shall
      contain any statement of fact, information or other matter which is
      inconsistent with the matters stated in Phase I Owner's representations in
      this Section 6.l(c), the Tenant Estoppel shall control and Phase I Owner
      shall have no liability for any claim based upon a breach of
      representation regarding such statement of fact, information or other
      matter contained in the Tenant Estoppel. Notwithstanding anything to the
      contrary contained in this Agreement, Phase I Owner does not represent or
      warrant that any particular Phase I Lease will be in force or effect at
      Closing or that the tenants under the Phase I Leases will have performed
      their obligations thereunder. The termination of any Phase I Lease prior
      to Closing by reason of the tenant's default shall not affect the
      obligations of Cornerstone under this Agreement in any manner or entitle
      Cornerstone to an abatement of or credit against the Purchase Price or
      give rise to any other claim on the part of Cornerstone. The Lease with
      Matas Corporation shall terminate as of Closing.

            (d) Lease Brokerage. Phase I Owner has paid all leasing commissions
      heretofore due and payable with respect to the Phase I Leases and, except
      as set forth on Exhibit M-1, (i) to Phase I Owner's knowledge, no person
      is entitled to any leasing commission in connection with the extension or
      renewal of any Phase I Lease or in connection with the exercise by any
      tenant of any expansion or extension option contained in any of the Phase
      I Leases, (ii) neither the Phase I Owner nor the Phase I Property is
      subject to any "protection list" or similar obligation with respect to the
      future leasing of the Phase I Property, and (iii) Phase I Owner has not
      received any notice of any valid claim for commission with respect to the
      Phase I Leases which has not been paid.

            (e) No Violations. To Phase I Owner's knowledge, Phase I Owner has
      not received prior to the Effective Date any written notification from any
      governmental or public authority (i) that the Phase I Property is in
      violation of any applicable fire, health, building, use, occupancy or
      zoning laws where such violation remains outstanding and, if unaddressed,
      would have a material adverse effect on the use of the Phase I Property as
      currently owned and operated or (ii) that any work is required to be done
      upon or in connection with the Phase I Property, where such work remains
      outstanding and, if unaddressed, would have a material adverse effect on
      the use of the Phase I Property as currently owned and operated.


                                       18
<PAGE>

            (f) Taxes and Assessments. True and complete copies of the most
      recent real estate tax bills for the Phase I Property have been delivered
      to Cornerstone.

            (g) Condemnation. No condemnation proceedings relating to the Phase
      I Property are pending or, to Phase I Owner's knowledge, threatened.

            (h) Insurance. Phase I Owner has not received any written notice
      from any insurance company or board of fire underwriters of any defects or
      inadequacies in or on the Phase I Property or any part or component
      thereof that would materially and adversely affect the insurability of the
      Phase I Property or cause any material increase in the premiums for
      insurance for the Phase I Property that have not been cured or repaired.

            (i) Environmental Matters. Except as set forth in any environmental
      assessment reports in Phase I Owner's possession and disclosed to
      Cornerstone or as otherwise disclosed to Cornerstone, to Phase I Owner's
      knowledge, Phase I Owner has received no written notification that any
      governmental or quasi-governmental authority has determined that there are
      any violations of environmental statutes, ordinances or regulations
      affecting the Phase I Property. Phase I Owner has delivered or made
      available to Cornerstone true and correct copies of all engineering,
      environmental and similar reports relating to the Phase I Property which
      are in Phase I Owner's possession. As used herein, "Hazardous Substances"
      means all hazardous or toxic materials, substances, pollutants,
      contaminants, or wastes currently identified as a hazardous substance or
      waste in the Comprehensive Environmental Response, Compensation and
      Liability Act of 1980 (commonly known as "CERCLA"), as amended, the
      Superfund Amendments and Reauthorization Act (commonly known as "SARA"),
      the Resource Conservation and Recovery Act (commonly known as "RCRA"), or
      any other federal, state or local legislation or ordinances applicable to
      the Property.

            (j) Operating Agreements. The Operating Agreements Schedule lists
      all service, maintenance, supply and management contracts affecting the
      Phase I Property in effect on the date hereof, and (i) except as set forth
      on the Operating Agreements Schedule, the Operating Agreements have not
      been modified or amended and are in full force and effect, (ii) Phase I
      Owner has delivered to Cornerstone true and complete copies of the
      Operating Agreements and (iii) except as set forth on the Operating
      Agreements Schedule, Phase I Owner has not sent written notice to any
      contractor under any Operating Agreement claiming such party to be in
      default, which default remains uncured.

            (k) Licenses and Permits. All licenses and permits from governmental
      authorities necessary for the operation or maintenance of the Phase I
      Property have been issued and, Phase I Owner has not received written
      notice of the termination or expiration thereof or any violations
      thereunder, and to the knowledge of Phase I Owner, the same are in full
      force and effect.

            (l) Employees. Phase I Owner does not directly employ any employees
      who


                                       19
<PAGE>

      work at the Phase I Property.

            (m) Rent Roll. Attached hereto as Exhibit N-1 is a true and correct
      rent roll for the Phase I Property (specifying the name of the tenant, the
      space leased, the amount of base rent payable, the tenant's share of
      escalations, the amount of any arrearages (and the age of such
      arrearages), and the commencement and expiration dates of each Phase I
      Lease).

            (n) Financial Statements. To Phase I Owner's knowledge, the
      financial statements of the Phase I Property heretofore delivered to
      Cornerstone by or on behalf of Phase I Owner are true and correct in all
      material respects as to the period to which they relate.

            (o) Beneficial Interest. Phase I Owner is the sole owner of the
      beneficial interest in the Illinois land trust which holds title to the
      Phase I Land and the Phase I Improvements, free and clear of any liens,
      encumbrances or rights of others, other than the holder of the Existing
      Loan which is secured by the Phase I Property.

      6.2 Representations and Warranties of Phase II Owner. Phase II Owner
hereby makes the following representations and warranties to Cornerstone as of
the Effective Date:

            (a) Organization; Authority; Binding Obligations; No Violation.
      Phase II Owner has been duly organized and is validly existing under the
      laws of the State of Illinois. Phase II Owner has full right, authority,
      power and capacity: (i) to enter into this Agreement and each agreement,
      document and instrument to be executed and delivered by Phase II Owner
      pursuant to this Agreement; and (ii) to carry out the transactions
      contemplated hereby and thereby. This Agreement and each agreement,
      document and instrument executed and delivered by Phase II Owner pursuant
      to this Agreement constitutes, or when executed and delivered will
      constitute, the legal, valid and binding obligation of Phase II Owner
      enforceable against Phase II Owner in accordance with their respective
      terms. The execution, delivery and performance of this Agreement and each
      agreement, document and instrument executed and delivered by Phase II
      Owner: (x) does not and will not violate Phase II Owner's organizational
      documents; (y) does not and will not violate any federal, state, local or
      other laws applicable to Phase II Owner or require Phase II Owner to
      obtain any approval, consent or waiver of, or make any filing with, any
      person or authority (governmental or otherwise) that has not been obtained
      or made or which does not remain in effect; and (z) does not and will not
      violate or conflict with, or result in a breach of, or constitute a
      default under, any notice, mortgage, contract or agreement to which Phase
      II Owner is a party, or by which Phase II Owner or any of its properties
      is bound, other than the documents evidencing and securing the Existing
      Loans.

            (b) Pending Actions. To Phase II Owner's knowledge, except as
      described on Exhibit J-2 hereto, there is no action, suit, arbitration,
      unsatisfied order or judgment, governmental investigation or proceeding
      pending against the Phase H Property or the


                                       20
<PAGE>

      transaction contemplated by this Agreement.

            (c) Phase II Leases. The Phase II Leases listed in the Phase II
      Lease Schedule constitute all of the leases, licenses and occupancy
      agreements affecting the Phase II Property to which the Phase II Owner is
      a party (either directly or by successor to ownership of the Building)
      and, except as set forth on the Phase II Lease Schedule or on Exhibit K-2
      attached hereto: (i) the Phase II Leases have not been modified or
      amended, (ii) as of the date hereof, to Phase II Owner's knowledge, the
      Leases are in full force and effect, and the Phase II Owner has not sent
      written notice to any tenant under a Lease claiming that the tenant is in
      default under its Lease, which default remains uncured, and the Phase II
      Owner has no knowledge of any such default, and (iii) Phase II Owner has
      delivered to Cornerstone, true and complete copies of the Phase 11 Leases.
      Exhibit L-2 attached hereto sets forth a true and correct schedule of all
      security deposits held by Phase II Owner pursuant to the Phase II Leases,
      specifying the tenant which deposited such security deposit. No interest
      is payable by Phase II Owner to any tenant with respect to such security
      deposits. To Phase II Owner's actual knowledge, there are no subleases
      under, or assignment of, the Phase II Leases except as set forth on the
      Phase II Lease Schedule. Except as set forth in the Phase II Leases, Phase
      II Owner has not granted to any person any right to acquire an interest in
      or to lease, or any right of first offer or first refusal with respect to,
      all or any portion of the Phase II Property. Except as set forth on
      Exhibit K-2, (A) each of the tenants under the Phase II Leases has taken
      possession of its respective demised premises and has commenced payment of
      rent, (B) all work allowances or other sums required to be paid as of the
      date hereof by the landlord under the Phase II Leases, and all work
      required to be performed by the landlord with respect to the preparation
      of such demised premises or the Phase II Property for the tenant's
      occupancy under the Phase II Leases, and any other work or other
      obligation required to be performed by the landlord under the Leases, and
      any other work or other obligation required to be performed by the
      landlord under the Leases, has been performed and/or paid for, (C) Phase
      II Owner has not received from any of the tenants under the Phase II
      Leases any written notice (i) claiming any default by the landlord under
      its Lease that has not been remedied, (ii) claiming that it is entitled to
      any offset, setoff or deduction against rent, (iii) disputing any rent
      computation that has not been resolved or (iv) advising Phase II Owner
      that it intends to vacate its premises upon the expiration of the term of
      its Phase II Lease or exercise any right to terminate its Phase II Lease,
      and (D) to Phase II Owner's knowledge, as Phase II Owner is not in default
      of any of its obligations under the Phase II Leases, no tenant under the
      Phase II Leases is entitled to any offset, setoff, or deduction against
      rent and there are no disputes regarding rent computations that have not
      been resolved. In the event that any Tenant Estoppel delivered to
      Cornerstone with respect to any Phase II Lease shall contain any statement
      of fact, information or other matter which is inconsistent with the
      matters stated in Phase II Owner's representations in this Section 6.l(c),
      the Tenant Estoppel shall control and Phase II Owner shall have no
      liability for any claim based upon a breach of representation regarding
      such statement of fact, information or other matter contained in the
      Tenant Estoppel. Notwithstanding anything to the contrary contained in
      this Agreement, Phase II Owner does


                                       21
<PAGE>

      not represent or warrant that any particular Phase 11 Lease will be in
      force or effect at Closing or that the tenants under the Phase II Leases
      will have performed their obligations thereunder. The termination of any
      Phase II Lease prior to Closing by reason of the tenant's default shall
      not affect the obligations of Cornerstone under this Agreement in any
      manner or entitle Cornerstone to an abatement of or credit against the
      Purchase Price or give rise to any other claim on the part of Cornerstone.

            (d) Lease Brokerage. Phase II Owner has paid all leasing commissions
      heretofore due and payable with respect to the Phase II Leases and, except
      as set forth on Exhibit M-2, (i) to Phase II Owner's knowledge, no person
      is entitled to any leasing commission in connection with the extension or
      renewal of any Phase II Lease or in connection with the exercise by any
      tenant of any expansion or extension option contained in any of the Phase
      II Leases, (ii) neither the Phase II Owner nor the Phase II Property is
      subject to any "protection list" or similar obligation with respect to the
      future leasing of the Phase II Property, and (iii) Phase II Owner has not
      received any notice of any valid claim for commission with respect to the
      Phase II Leases which has not been paid.

            (e) No Violations. To Phase II Owner's knowledge, Phase II Owner has
      not received prior to the Effective Date any written notification from any
      governmental or public authority (i) that the Phase II Property is in
      violation of any applicable fire, health, building, use, occupancy or
      zoning laws where such violation remains outstanding and, if unaddressed,
      would have a material adverse effect on the use of the Phase II Property
      as currently owned and operated or (ii) that any work is required to be
      done upon or in connection with the Phase II Property, where such work
      remains outstanding and, if unaddressed, would have a material adverse
      effect on the use of the Phase II Property as currently owned and
      operated.

            (f) Taxes and Assessments. True and complete copies of the most
      recent real estate tax bills for the Phase II Property have been delivered
      to Cornerstone.

            (g) Condemnation. No condemnation proceedings relating to the Phase
      II Property are pending or, to Phase II Owner's knowledge, threatened.

            (h) Insurance. Phase II Owner has not received any written notice
      from any insurance company or board of fire underwriters of any defects or
      inadequacies in or on the Phase II Property or any part or component
      thereof that would materially and adversely affect the insurability of the
      Phase II Property or cause any material increase in the premiums for
      insurance for the Phase II Property that have not been cured or repaired.

            (i) Environmental Matters. Except as set forth in any environmental
      assessment reports in Phase II Owner's possession and disclosed to
      Cornerstone or as otherwise disclosed to Cornerstone, to Phase II Owner's
      knowledge, Phase II Owner has received no written notification that any
      governmental or quasi-governmental authority has determined


                                       22
<PAGE>

      that there are any violations of environmental statutes, ordinances or
      regulations affecting the Phase II Property. Phase II Owner has delivered
      or made available to Cornerstone true and correct copies of all
      engineering, environmental and similar reports relating to the Phase II
      Property which are in Phase II Owner's possession.

            (j) Operating Agreements. The Operating Agreements Schedule lists
      all service, maintenance, supply and management contracts affecting the
      Phase II Property in effect on the date hereof, and (i) except as set
      forth on the Operating Agreements Schedule, the Operating Agreements have
      not been modified or amended and are in full force and effect, (ii) Phase
      II Owner has delivered to Cornerstone true and complete copies of the
      Operating Agreements and (iii) except as set forth on the Operating
      Agreements Schedule, Phase II Owner has not sent written notice to any
      contractor under any Operating Agreement claiming such party to be in
      default, which default remains uncured.

            (k) Licenses and Permits. All licenses and permits from governmental
      authorities necessary for the operation or maintenance of the Phase II
      Property have been issued and, Phase II Owner has not received written
      notice of the termination or expiration thereof or any violations
      thereunder, and to the knowledge of Phase II Owner, the same are in full
      force and effect.

            (l) Employees. Phase II Owner does not directly employ any employees
      who work at the Phase II Property.

            (m) Rent Roll. Attached hereto as Exhibit N-2 is a true and correct
      rent roll for the Phase II Property (specifying the name of the tenant,
      the space leased, the amount of base rent payable, the tenant's share of
      escalations, the amount of any arrearages (and the age of such
      arrearages), and the commencement and expiration dates of each Phase II
      Lease).

            (n) Financial Statements. To Phase II Owner's knowledge, the
      financial statements of the Phase II Property heretofore delivered to
      Cornerstone by or on behalf of Phase II Owner are true and correct in all
      material respects as to the period to which they relate.

            (o) Beneficial Interest. Phase II Owner is the sole owner of the
      beneficial interest in the Illinois land trust which holds title to the
      Phase II Land and the Phase II Improvements, free and clear of any liens,
      encumbrances or rights of others, other than the holder of the Existing
      Loan which is secured by the Phase II Property.

      6.3 Intentionally Omitted.

      6.4 Knowledge Defined. References to the "knowledge" of Phase I Owner and
Phase II Owner shall refer only to the actual knowledge of Myron Levin and Arvin
L. Reiger, and shall


                                       23
<PAGE>

not be construed, by imputation or otherwise, to refer to the knowledge of Phase
I Owner or Phase II Owner or any affiliate of either of them, to any property
manager, or to any other officer, agent, manager, representative or employee of
Phase I Owner or Phase II Owner or any affiliate or to impose upon such persons
any duty to investigate the matter to which such actual knowledge, or the
absence thereof, pertains.

      6.5 Survival of Contributors' Representations and Warranties. The
representations and warranties of Contributors set forth in Sections 6.1 and
6.2, as updated by the certificates of Contributors to be delivered to
Cornerstone at Closing in accordance with Section 5.2(g) hereof, and the
representations and warranties of Manager set forth in Section 6.3, shall
survive Closing for a period of one year. No claim for a breach of any
representation or warranty of Contributors shall be actionable or payable (a) if
the breach in question results from or is based on a condition, state of facts
or other matter which was known to Cornerstone prior to Closing and contained in
any written documents, reports, instruments or correspondence delivered to
Cornerstone by Contributors or made available by Contributors for Cornerstone's
review or prepared by any of Cornerstone's consultants or employees, (b) unless
the valid claims for all such breaches collectively aggregate more than Three
Hundred Thousand Dollars ($300,000), in which event the full amount of such
claims shall be actionable, and (c) unless written notice containing a
description of the specific nature of such breach shall have been given by
Cornerstone to Contributors and Manager prior to the expiration of said one year
period and an action shall have been commenced by Cornerstone against
Contributors or Manager within six months thereafter.

      6.6 Covenants of Contributors. Contributors hereby covenant with
Cornerstone as follows:

            (a) From the Effective Date hereof until the Closing or earlier
      termination of this Agreement, Contributors shall operate and maintain the
      Property in a manner generally consistent with the manner in which
      Contributors have operated and maintained the Property prior to the date
      hereof.

            (b) Contributors shall use reasonable efforts (but without
      obligation to incur any cost or expense) to obtain and deliver to
      Cornerstone prior to Closing, a written estoppel certificate in the form
      of Exhibit E attached hereto and made a part hereof as modified for
      specific lease terms and circumstances signed by each tenant occupying
      space in the Improvements. The signed certificates are referred to herein
      as the "Tenant Estoppels". In the event that any such Tenant Estoppels are
      not so obtained and delivered, Phase I Owner or Phase II Owner, as
      applicable, shall have the right (but not the obligation) to execute and
      deliver to Purchaser at the Closing a certificate (hereinafter referred to
      as a "Seller Estoppel") in favor of the Operating Partnership warranting
      that the items set forth in Exhibit E with respect to tenants (other than
      the Major Tenants) under Leases for which a Tenant Estoppel has not been
      obtained are true, provided that: (i) the Seller Estoppels shall survive
      the Closing and shall automatically expire and terminate on the first
      anniversary of the Closing if no claim has theretofore been made
      thereunder or earlier upon receipt by the


                                       24
<PAGE>

      Operating Partnership of a Tenant Estoppel from the applicable tenant, and
      (ii) any statement of facts which is not contained in such tenant's Lease
      shall be made to the knowledge of Phase I Owner or Phase II Owner, as
      applicable.

            (c) A copy of any renewal or expansion of an existing Lease or of
      any new Lease which Contributor wishes to execute between the Effective
      Date and the date of Closing will be submitted to Cornerstone prior to
      execution by Contributor. Cornerstone agrees to notify Contributor in
      writing within five (5) business days after its receipt thereof of either
      its approval (which approval will not be unreasonably withheld) or
      disapproval, including all Tenant Inducement Costs and leasing commissions
      to be incurred in connection therewith. In the event Cornerstone fails to
      notify Contributor in writing of its approval or disapproval within the
      five (5) day time period for such purpose set forth above, such failure
      shall be deemed the approval by Cornerstone.

      6.7 Representations and Warranties of Cornerstone. Cornerstone hereby
represents and warrants to Contributors and Manager:

            (a) Authority; Binding Obligations; and No Violation. Cornerstone
      has full right, authority, power and capacity: (i) to enter into this
      Agreement, the Partnership Agreement and each other agreement, document
      and instrument to be executed and delivered by or on behalf of Cornerstone
      and the Operating Partnership pursuant to this Agreement; and (ii) to
      carry out the transactions contemplated hereby and thereby. This
      Agreement, the Partnership Agreement and each other agreement, document
      and instrument executed and delivered by or on behalf of Cornerstone and
      the Operating Partnership pursuant to this Agreement constitutes, or when
      executed and delivered will constitute, the legal, valid and binding
      obligation of Cornerstone and the Operating Partnership, as applicable,
      each enforceable in accordance with their respective terms. The execution,
      delivery and performance of this Agreement, the Partnership Agreement and
      each other agreement, document and instrument executed and delivered by or
      on behalf of Cornerstone and the Operating Partnership: (x) does not and
      will not violate Cornerstone's or the Operating Partnership's
      organizational documents; (y) does not and will not violate any foreign,
      federal, state, local or other laws applicable to Cornerstone or the
      Operating Partnership or require Cornerstone or the Operating Partnership
      to obtain any approval, consent or waiver of, or make any filing with, any
      person or authority (governmental or otherwise) that has not been obtained
      or made or which does not remain in effect; and (z) does not and will not
      violate or conflict with, or result in a breach of, or constitute a
      default under, any note, mortgage, contract or agreement to which
      Cornerstone or the Operating Partnership is a party, or by which
      Cornerstone or Operating Partnership or any of their properties is bound.

            (b) Units and Shares. The Units to be issued upon contribution of
      the Property and the Management Agreements will have been duly authorized
      and validly issued, free and clear of all mortgages, pledges, liens,
      security interests, encumbrances and restrictions of every nature, except
      as provided in this Agreement, the Partnership Agreement and the


                                       25
<PAGE>

      Registration Rights Agreement. The shares of common stock of Cornerstone
      that may be issued upon conversion of the Units under the terms of the
      Partnership Agreement will have been duly authorized and when issued in
      accordance with the terms of the Partnership Agreement, will be validly
      issued and fully paid and non assessable.

            (c) No Litigation. There is no action, suit, arbitration,
      unsatisfied order or judgment, government investigation or proceeding
      pending against Cornerstone which, if adversely determined, could
      individually or in the aggregate materially interfere with the
      consummation of the transaction contemplated by this Agreement.

            (d) REIT Qualification. Cornerstone is qualified, has been qualified
      since the year ended December 31, 1983, and has been operating since the
      beginning of the current fiscal year, in a manner that would continue to
      permit it to be qualified, and intends to operate so it continues to be
      qualified, as a real estate investment trust under Section 856 et seq. of
      the Code.

            (e) Tax Returns and Financial Statements. Cornerstone's federal
      income tax returns and financial statements for the calendar years 1995
      and 1996, have been provided to Contributors and together with all interim
      financial statements for 1997 heretofore furnished to Contributors are
      true, correct and complete.

            (f) No Material Adverse Change. Since September 30, 1997, there has
      not been any material adverse change in the business, operations,
      properties, assets or condition of Cornerstone or any event, condition, or
      contingency that is likely to result in such a material adverse change.

            (g) SEC Reports. Cornerstone has filed all forms, reports and
      documents required to be filed with the SEC since December 31, 1996,
      including (i) its Annual Report on Form 10-K for the fiscal year ended
      December 31, 1996, (ii) its Quarterly Reports on Form 10-Q for the periods
      ended March 31, June 30 and September 30, 1997, (iii) all other reports or
      registration statements filed by Cornerstone with the SEC since December
      31, 1996, and (iv) all amendments and supplements to all such reports and
      registration statements filed by Contributor with the SEC. All such
      required forms, reports and documents (including those enumerated in
      clauses (i) through (iv) of the preceding sentence) are referred to herein
      as the "SEC Reports".

            (h) Operating Partnership. Upon formation, the Operating Partnership
      (i) will be a limited partnership duly organized, validly existing and in
      good standing under the laws of the State of Delaware, (ii) will have all
      requisite partnership power and authority to own, operate, lease and
      encumber its properties and conduct the business for which it is formed,
      and (iii) will be duly qualified to do business and be in good standing in
      each jurisdiction in which the ownership of its property or the conduct of
      its business requires such qualification. The sole general partner of the
      Operating Partnership will be Cornerstone.


                                       26
<PAGE>

      6.8 Survival of Cornerstone's Representations and Warranties. The
representations and warranties of Cornerstone set forth in Section 6.7 shall
survive Closing and shall be a continuing representation and warranty without
limitation.

      6.9 Covenants of Cornerstone. Cornerstone hereby covenants with
Contributors as follows:

            (a) Cornerstone shall, in connection with its investigation of the
      Property during the Inspection Period, inspect the Property for the
      presence of Hazardous Substances (as defined in Section 6. 1 (i) hereof),
      and shall furnish to Contributors copies of any reports received by
      Cornerstone in connection with any such inspection. Cornerstone shall also
      furnish to Contributors copies of any other reports received by
      Cornerstone relating to any other inspections of the Property conducted on
      Cornerstone's behalf, if any (including, specifically, without limitation,
      any reports analyzing compliance of the Property with the provisions of
      the Americans with Disabilities Act ("ADA"), 42 U.S.C. ss.12101, et seq.,
      if applicable).

      6.10 Investment Representations and Warranties. Each Contributor
represents and warrants as follows:

            (a) It is an "accredited investor" within the meaning of Rule 501
      promulgated under Regulation D of the Securities Act of 1933, as amended
      (the "Securities Act"). It understands the risks of, and other
      considerations relating to, the purchase of the Units. It, by reason of
      its business and financial experience, together with the business and
      financial experience of those persons, if any, retained by it to represent
      or advise it with respect to its investment in the Units, has such
      knowledge, sophistication and experience in financial and business matters
      and in making investment decisions of this type that it is capable of
      evaluating the merits and risks of an investment in the Operating
      Partnership and of making an informed investment decision, (ii) is capable
      of protecting its own interest or has engaged representatives or advisors
      to assist it in protecting its interest and (iii) is capable of bearing
      the economic risk of such investment.

            (b) The Units to be issued to it will be acquired by it for its own
      account for investment only and not with a view to, or with any intention
      of, a distribution or resale thereof, in whole or in part, or the grant of
      any participation therein.

            (c) Each Contributor acknowledges that (i) the Units to be issued to
      it have not been registered under the Securities Act or state securities
      laws by reason of a specific exemption or exemptions from registration
      under the Securities Act and applicable state securities laws, (ii)
      Cornerstone's and the Operating Partnership's reliance on such exemptions
      is predicated in part on the accuracy and completeness of the
      representations and warranties of it contained herein, (iii) such Units,
      therefore, cannot be resold unless registered under the Securities Act and
      applicable state securities laws, or unless an


                                       27
<PAGE>

      exemption from registration is available, (iv) there is no public market
      for such Units, and (v) the Operating Partnership has no obligation or
      intention to register such Units for resale under the Securities Act or
      any state securities laws or to take any action that would make available
      any exemption from the registration requirements of such laws. Each
      Contributor hereby acknowledges that because of the restrictions on
      transfer or assignment of such Units to be issued hereunder which are set
      forth in this Agreement and in the Partnership Agreement, it may have to
      bear the economic risk of the investment commitment evidenced by this
      Agreement and any Units purchased hereby for an indefinite period of time,
      although (x) under the terms of the Partnership Agreement, Units may be
      converted into common stock of Cornerstone and (y) the holder of any such
      common stock issued upon a presentation of Units for conversion will be
      afforded certain rights to have such common stock registered for resale
      under the Securities Act or applicable state securities laws under the
      Registration Rights Agreement.

            (d) Contributors are sophisticated investors and their respective
      decisions to acquire the Units (and upon conversion Cornerstone common
      stock) are based upon their independent evaluations of publicly available
      information regarding the financial condition, operations, properties and
      prospects of Cornerstone, and other facts that Contributors deem material
      to Contributors' decision. Contributors have not relied in entering into
      this Agreement upon any oral or written information from Cornerstone, or
      any of its respective employees, affiliates, agents or representatives,
      other than the representations and warranties of Cornerstone contained
      herein and Cornerstone's periodic public filings with the Securities
      Exchange Commission. Contributors have conducted such due diligence and
      analysis as Contributors deemed necessary, proper or appropriate in order
      to make a complete informed decision with respect to its acquisition of
      the Units. Contributors acknowledge that the Units may have limited or no
      liquidity until converted to shares of Cornerstone common stock.

                                  ARTICLE VII

                                    DEFAULT

      7.1 Default by Cornerstone. If Cornerstone defaults for any reason other
than Contributors' default or the permitted termination of this Agreement by
Contributors or Cornerstone as herein expressly provided, Contributors shall be
entitled, as their sole remedy, to terminate this Agreement and receive the
Earnest Money as liquidated damages for the breach of this Agreement, it being
agreed between the parties hereto that the actual damages to Contributors in the
event of such breach are impractical to ascertain and the amount of the Earnest
Money is a reasonable estimate thereof.

      7.2 Default by Contributors or Manager. In the event that Contributors
fail to consummate this Agreement for any reason other than Cornerstone's
default or the permitted termination of this Agreement by Contributors or
Cornerstone as herein expressly provided,


                                       28
<PAGE>

Cornerstone shall be entitled, as its sole remedy, either (a) to receive the
return of the Earnest Money, which return shall operate to terminate this
Agreement and release Contributors from any and all liability hereunder, or (b)
to enforce specific performance of Contributors' obligations to execute the
documents required to contribute the Property to the Operating Partnership, it
being understood and agreed that the remedy of specific performance shall not be
available to enforce any other obligations of Contributors hereunder.
Cornerstone expressly waives its rights to seek damages in the event of
Contributors' default hereunder. Cornerstone shall be deemed to have elected to
terminate this Agreement and receive back the Earnest Money if Cornerstone fails
to file suit for specific performance against Contributors in a court having
jurisdiction in the county and state in which the Property is located, on or
before sixty (60) days following the date upon which Closing was to have
occurred.

                                  ARTICLE VIII

                                  RISK OF LOSS

      8.1 Minor Damage. In the event of loss or damage to the Property or any
portion thereof which is not "major" (as hereinafter defined), this Agreement
shall remain in full force and effect provided Contributors perform any
necessary repairs or, at Contributors' option, assigns to the Operating
Partnership all of Contributors' right, title and interest to any claims and
proceeds Contributors may have with respect to any casualty insurance policies
or condemnation awards relating to the premises in question. In the event that
Contributors elect to perform repairs upon the Property, Contributors shall use
reasonable efforts to complete such repairs promptly and the date of Closing
shall be extended a reasonable time in order to allow for the completion of such
repairs. If Contributors elect to assign a casualty claim to the Operating
Partnership, the Property Acquisition Value shall be reduced by an amount equal
to the deductible amount under Contributors' insurance policy. Upon Closing,
full risk of loss with respect to the Property shall pass to the Operating
Partnership.

      8.2 Major Damage. In the event of a "major" loss or damage, either
Contributors or Cornerstone may terminate this Agreement by written notice to
the other party, in which event the Earnest Money shall be returned to
Cornerstone. If neither Contributors nor Cornerstone elects to terminate this
Agreement within ten (10) days after Contributors send Cornerstone written
notice of the occurrence of major loss or damage, then Contributors and
Cornerstone shall be deemed to have elected to proceed with Closing, in which
event Contributors shall, at Contributors' option, either (a) perform any
necessary repairs, or (b) assign to the Operating Partnership all of
Contributors' right, title and interest to any claims and proceeds Contributors
may have with respect to any casualty insurance policies or condemnation awards
relating to the premises in question. In the event that Contributors elect to
perform repairs upon the Property, Contributors shall use reasonable efforts to
complete such repairs promptly and the date of Closing shall be extended a
reasonable time in order to allow for the completion of such repairs. If
Contributors elect to assign a casualty claim to the Operating Partnership, the
Property Acquisition Value shall be reduced by


                                       29
<PAGE>

an amount equal to the deductible amount under Contributors' insurance policy.
Upon Closing, full risk of loss with respect to the Property shall pass to the
Operating Partnership.

      8.3 Definition of "Major" Loss or Damage. For purposes of Sections 8.1 and
8.2, "major" loss or damage refers to the following: (i) loss or damage to the
Property or any portion thereof such that the cost of repairing or restoring the
premises in question to a condition substantially identical to that of the
premises in question prior to the event of damage would be, in the opinion of an
architect selected by Contributors and reasonably approved by Cornerstone, equal
to or greater than Seven Million Five Hundred Thousand and No/100 Dollars
($7,500,000.00), and (ii) any loss due to a condemnation which permanently and
materially impairs the current use of the Property. If Cornerstone does not give
notice to Contributors of Cornerstone's reasons for disapproving an architect
within five (5) business days after receipt of notice of the proposed architect,
Cornerstone shall be deemed to have approved the architect selected by
Contributors.

                                   ARTICLE IX

                                  COMMISSIONS

      9.1 Brokerage Commissions. Each party represents to the other that there
has been no broker or finder engaged in connection with the transaction
contemplated hereby other than Julian Taft & Downey, Inc. (the "Broker").
Contributors agree to pay any fee payable to Broker pursuant to a separate
agreement between Contributors and Broker. Each party agrees that should any
claim be made for brokerage commissions or finder's fees by any broker or finder
other than the Broker by, through or on account of any acts of said party or its
representatives, said party will indemnify and hold the other party free and
harmless from and against any and all loss, liability, cost, damage and expense
in connection therewith. The provisions of this paragraph shall survive Closing.

                                   ARTICLE X

                            DISCLAIMERS AND WAIVERS

      10.1 No Reliance on Documents. Except as expressly stated herein,
Contributors make no representation or warranty as to the truth, accuracy or
completeness of any materials, data or information delivered by Contributors to
Cornerstone in connection with the transaction contemplated hereby. Cornerstone
acknowledges and agrees that all materials, data and information delivered by
Contributors to Cornerstone in connection with the transaction contemplated
hereby are provided to Cornerstone as a convenience only and that any reliance
on or use of such materials, data or information by Cornerstone shall be at the
sole risk of Cornerstone, except as otherwise expressly stated herein. Without
limiting the generality of the foregoing provisions, Cornerstone acknowledges
and agrees that (a) any environmental or other report with respect to the
Property which is delivered by Contributors to Cornerstone shall be for general
informational purposes only,


                                       30
<PAGE>

(b) Cornerstone shall not have any right to rely on any such report delivered by
Contributors to Cornerstone, but rather will rely on its own inspections and
investigations of the Property and any reports commissioned by Cornerstone with
respect thereto, and (c) neither Contributors, any affiliate of Contributors,
Manager nor the person or entity which prepared any such report delivered by
Contributors to Cornerstone shall have any liability to Cornerstone for any
inaccuracy in or omission from any such report.

      10.2 DISCLAIMERS. EXCEPT AS EXPRESSLY SET FORTH IN THIS AGREEMENT, IT IS
UNDERSTOOD AND AGREED THAT CONTRIBUTORS ARE NOT MAKING AND HAVE NOT AT ANY TIME
MADE ANY WARRANTIES OR REPRESENTATIONS OF ANY KIND OR CHARACTER, EXPRESSED OR
IMPLIED, WITH RESPECT TO THE PROPERTY, INCLUDING, BUT NOT LIMITED TO, ANY
WARRANTIES OR REPRESENTATIONS AS TO HABITABILITY, MERCHANTABILITY, FITNESS FOR A
PARTICULAR PURPOSE, TITLE, ZONING, TAX CONSEQUENCES, LATENT OR PATENT PHYSICAL
OR ENVIRONMENTAL CONDITION, UTILITIES, OPERATING HISTORY OR PROJECTIONS,
VALUATION, GOVERNMENTAL APPROVALS, THE COMPLIANCE OF THE PROPERTY WITH
GOVERNMENTAL LAWS, THE TRUTH, ACCURACY OR COMPLETENESS OF THE PROPERTY DOCUMENTS
OR ANY OTHER INFORMATION PROVIDED BY OR ON BEHALF OF CONTRIBUTORS TO
CORNERSTONE, OR ANY OTHER MATTER OR THING REGARDING THE PROPERTY. CORNERSTONE
ACKNOWLEDGES AND AGREES THAT UPON CLOSING CONTRIBUTORS SHALL CONVEY TO OPERATING
PARTNERSHIP AND OPERATING PARTNERSHIP SHALL ACCEPT THE PROPERTY "AS IS, WHERE
IS, WITH ALL FAULTS", EXCEPT TO THE EXTENT EXPRESSLY PROVIDED OTHERWISE IN THIS
AGREEMENT. CORNERSTONE HAS NOT RELIED AND WILL NOT RELY ON, AND CONTRIBUTORS ARE
NOT LIABLE FOR OR BOUND BY, ANY EXPRESSED OR IMPLIED WARRANTIES, GUARANTIES,
STATEMENTS, REPRESENTATIONS OR INFORMATION PERTAINING TO THE PROPERTY OR
RELATING THERETO (INCLUDING SPECIFICALLY, WITHOUT LIMITATION, PROPERTY
INFORMATION PACKAGES DISTRIBUTED WITH RESPECT TO THE PROPERTY) MADE OR FURNISHED
BY CONTRIBUTORS, THE MANAGER OF THE PROPERTY, OR ANY REAL ESTATE BROKER OR AGENT
REPRESENTING OR PURPORTING TO REPRESENT CONTRIBUTORS, TO WHOMEVER MADE OR GIVEN,
DIRECTLY OR INDIRECTLY, ORALLY OR IN WRITING, UNLESS SPECIFICALLY SET FORTH IN
THIS AGREEMENT. CORNERSTONE REPRESENTS TO CONTRIBUTORS THAT CORNERSTONE HAS
CONDUCTED, OR WILL CONDUCT PRIOR TO CLOSING, SUCH INVESTIGATIONS OF THE
PROPERTY, INCLUDING BUT NOT LIMITED TO, THE PHYSICAL AND ENVIRONMENTAL
CONDITIONS THEREOF, AS CORNERSTONE DEEMS NECESSARY TO SATISFY ITSELF AS TO THE
CONDITION OF THE PROPERTY AND THE EXISTENCE OR NONEXISTENCE OR CURATIVE ACTION
TO BE TAKEN WITH RESPECT TO ANY HAZARDOUS OR TOXIC SUBSTANCES ON OR DISCHARGED
FROM THE PROPERTY, AND WILL RELY SOLELY UPON SAME AND NOT UPON ANY INFORMATION
PROVIDED BY OR ON BEHALF OF CONTRIBUTORS OR THEIR AGENTS OR EMPLOYEES WITH
RESPECT THERETO, OTHER THAN SUCH REPRESENTATIONS,


                                       31
<PAGE>

WARRANTIES AND COVENANTS OF CONTRIBUTORS AS ARE EXPRESSLY SET FORTH IN THIS
AGREEMENT. UPON CLOSING, THE OPERATING PARTNERSHIP SHALL ASSUME THE RISK THAT
ADVERSE MATTERS, INCLUDING BUT NOT LIMITED TO, CONSTRUCTION DEFECTS AND ADVERSE
PHYSICAL AND ENVIRONMENTAL CONDITIONS, MAY NOT HAVE BEEN REVEALED BY
CORNERSTONE'S INVESTIGATIONS, AND CORNERSTONE AND OPERATING PARTNERSHIP, UPON
CLOSING, SHALL BE DEEMED TO HAVE WAIVED, RELINQUISHED AND RELEASED CONTRIBUTORS
(AND CONTRIBUTORS' PARTNERS AND THEIR RESPECTIVE OFFICERS, DIRECTORS,
SHAREHOLDERS, EMPLOYEES AND AGENTS) FROM AND AGAINST ANY AND ALL CLAIMS,
DEMANDS, CAUSES OF ACTION (INCLUDING CAUSES OF ACTION IN TORT), LOSSES, DAMAGES,
LIABILITIES, COSTS AND EXPENSES (INCLUDING ATTORNEYS' FEES AND COURT COSTS) OF
ANY AND EVERY KIND OR CHARACTER, KNOWN OR UNKNOWN, WHICH CORNERSTONE OR
OPERATING PARTNERSHIP MIGHT HAVE ASSERTED OR ALLEGED AGAINST CONTRIBUTORS (AND
CONTRIBUTORS' PARTNERS AND THEIR RESPECTIVE OFFICERS, DIRECTORS, SHAREHOLDERS,
EMPLOYEES AND AGENTS) AT ANY TIME BY REASON OF OR ARISING OUT OF ANY LATENT OR
PATENT CONSTRUCTION DEFECTS OR PHYSICAL CONDITIONS, VIOLATIONS OF ANY APPLICABLE
LAWS (INCLUDING, WITHOUT LIMITATION, ANY ENVIRONMENTAL LAWS) AND ANY AND ALL
OTHER ACTS, OMISSIONS, EVENTS, CIRCUMSTANCES OR MATTERS REGARDING THE PROPERTY.
CORNERSTONE AGREES THAT SHOULD ANY CLEANUP, REMEDIATION OR REMOVAL OF HAZARDOUS
SUBSTANCES OR OTHER ENVIRONMENTAL CONDITIONS ON THE PROPERTY BE REQUIRED AFTER
THE DATE OF CLOSING, SUCH CLEAN-UP, REMOVAL OR REMEDIATION SHALL BE THE
RESPONSIBILITY OF AND SHALL BE PERFORMED AT THE SOLE COST AND EXPENSE OF
OPERATING PARTNERSHIP.

      10.3 Effect and Survival of Disclaimers. Contributors and Cornerstone
acknowledge that the compensation to be paid to Contributors for the Property
takes into account that the Property is being sold subject to the provisions of
this Article IX. Contributors and Cornerstone agree that the provisions of this
Article IX shall survive Closing.

                                   ARTICLE XI

             CONTINUING AGREEMENTS AMONG CONTRIBUTORS, CORNERSTONE
                     AND OPERATING PARTNERSHIP; TAX MATTERS

      11.1 Disposition of Property. Cornerstone agrees that the Operating
Partnership shall not sell or dispose of the Land and Improvements prior to the
date (the "End Date") which is the earlier to occur of (a) the tenth anniversary
of the Closing, and (b) date on which Units aggregating less than 25% of the
number of Units distributed at Closing are held by the Contributors (or their
constituent partners), without the prior written consent of the then holders of
the Units, except in


                                       32
<PAGE>

connection with a like-kind exchange under Section 1031 of the Internal Revenue
Code of 1986, as amended (the "Code") or other disposition that pursuant to a
nonrecognition provision in the Code does not result in the current recognition
of any gain to said holders of the Units.

      11.2 Maintenance of Operating Partnership Loan. Cornerstone agrees that
the Operating Partnership shall keep outstanding the Operating Partnership Loan
in the principal amount of no less than $80,000,000 until the End Date.

      11.3 Conversion of Units. Contributors shall not convert any of the Units
into shares of Cornerstone for a period of one year from the Closing (the
"Lock-Up Period").

      11.4 Section 704(c) Allocation. Cornerstone and Contributors agree that,
for purposes of Section 704(c) of the Internal Revenue Code of 1986 as amended
(the "Code"), the consideration payable to Contributors as set forth in this
Agreement will be allocated among the various components of the Property in a
manner to be reasonably agreed upon by Cornerstone and Contributors prior to the
Closing. The method under Section 704(c) of the Code used by Cornerstone with
respect to the Property to adjust for discrepancies between the agreed upon
value of the various components of the Property and the adjusted tax basis of
such components will be the "traditional method," as set forth in Treasury
Regulation ss.1.704-3(c)(1).

      11.5 Allocation of Operating Partnership Loan. If requested in writing by
Contributors, the Operating Partnership Loan shall be split between the Phase I
Property and the Phase II Property in such amount as Contributors reasonably
request. Each portion of the Operating Partnership Loan may be cross defaulted
and cross collateralized with the other portion. In addition, the Contributors'
basis in the Operating Partnership Loan shall be allocated to each Contributor,
or to such Contributor's partners (or to partners of such partners), in
accordance with written instructions provided to the Operating Partnership by
Contributors, which allocation shall be subject to the approval of the Operating
Partnership, which approval will not be unreasonably withheld.

                                  ARTICLE XII

                                 MISCELLANEOUS

      12.1 Confidentiality. Cornerstone and its representatives shall hold in
strictest confidence all data and information obtained with respect to
Contributor or its business, whether obtained before or after the execution and
delivery of this Agreement, and shall not disclose the same to others; provided,
however, that it is understood and agreed that Cornerstone may disclose such
data and information as required by law (including securities laws) and to the
employees, consultants, accountants and attorneys of Cornerstone provided that
such persons agree in writing to treat such data and information confidentially.
In the event this Agreement is terminated or Cornerstone fails to perform
hereunder, Cornerstone shall promptly return to Contributors any statements,
documents, schedules, exhibits or other written information obtained from
Contributors in connection with this


                                       33
<PAGE>

Agreement or the transaction contemplated herein. It is understood and agreed
that, with respect to any provision of this Agreement which refers to the
termination of this Agreement and the return of the Earnest Money to
Cornerstone, such Earnest Money shall not be returned to Cornerstone unless and
until Cornerstone has fulfilled its obligation to return to Contributors the
materials described in the preceding sentence. In the event of a breach or
threatened breach by Cornerstone or its agents or representatives of this
Section 12.1, Contributors shall be entitled to an injunction restraining
Cornerstone or its agents or representatives from disclosing, in whole or in
part, such confidential information. Nothing herein shall be construed as
prohibiting Contributors from pursuing any other available remedy at law or in
equity for such breach or threatened breach. The provisions of this Section 12.1
shall survive Closing.

      12.2 Public Disclosure. Prior to Closing, any release to the public of
information with respect to the transaction contemplated herein or any matters
set forth in this Agreement, other than any disclosure required by securities
laws, will be made only in the form approved by Cornerstone and Contributors and
their respective counsel.

      12.3 Discharge of Obligations. The acceptance of the Deeds by the
Operating Partnership shall be deemed to be a full performance and discharge of
every representation and warranty made by Contributors herein and every
agreement and obligation on the part of Contributors to be performed pursuant to
the provisions of this Agreement, except those which are herein specifically
stated to survive Closing.

      12.4 Assignment. Cornerstone may not assign its rights under this
Agreement, other than to the Operating Partnership, without first obtaining
Contributors' written approval, which approval may be given or withheld in
Contributor's sole discretion.

      12.5 Notices. Any notice pursuant to this Agreement shall be given in
writing by (a) personal delivery, or (b) reputable overnight delivery service
with proof of delivery, or (c) United States Mail, postage prepaid, registered
or certified mail, return receipt requested, or (d) legible facsimile
transmission sent to the intended addressee at the address set forth below, or
to such other address or to the attention of such other person as the addressee
shall have designated by written notice sent in accordance herewith, and shall
be deemed to have been given either at the time of personal delivery, or, in the
case of expedited delivery service or mail, as of the date of first attempted
delivery at the address and in the manner provided herein, or, in the case of
facsimile transmission, as of the date of the facsimile transmission provided
that an original of such facsimile is also sent to the intended addressee by
means described in clauses (a), (b) or (c) above. Unless changed in accordance
with the preceding sentence, the addresses for notices given pursuant to this
Agreement shall be as follows:


                                       34
<PAGE>

                                    If to Contributors:

                                    c/o Matas Corporation
                                    500 Lake Cook Road
                                    Deerfield, Illinois  60015
                                    Attn.:  Myron Levin
                                    TELECOPY:  (847) 948-9364

                  with a copy to:   Katten Muchin & Zavis
                                    525 West Monroe Street
                                    Suite 1600
                                    Chicago, Illinois  60661-3693
                                    Attn.:  Nina B. Matis, Esq.
                                    TELECOPY:  (312) 902-1061

                  If to Purchaser:  Cornerstone Properties, Inc.
                                    126 East 56th Street
                                    New York, New York  10022
                                    Attn.: Francis H. Shields, Jr.
                                    TELECOPY:  (212) 605-7199

                  with a copy to:   Shearman & Sterling
                                    599 Lexington Avenue
                                    New York, NY  10022
                                    Attn.:  Timothy G. Little, Esq.
                                    TELECOPY:  (212) 848-7179

      12.6 Intentionally Omitted.

      12.7 Modifications. This Agreement cannot be changed orally, and no
executory agreement shall be effective to waive, change, modify or discharge it
in whole or in part unless such executory agreement is in writing and is signed
by the parties against whom enforcement of any waiver, change, modification or
discharge is sought.

      12.8 Tenant Notification Letters. Operating Partnership shall deliver to
each and every tenant of the Property under a Lease thereof a signed statement
acknowledging Operating Partnership's receipt and responsibility for each
tenant's security deposit (to the extent delivered by Contributors to Operating
Partnership at Closing), if any, all in compliance with and pursuant to the
applicable provisions of applicable law. The provisions of this paragraph shall
survive Closing.

      12.9 Calculation of Time Periods. Unless otherwise specified, in computing
any period of time described in this Agreement, the day of the act or event
after which the designated period of time begins to run is not to be included
and the last day of the period so computed is to be


                                       35
<PAGE>

included, unless such last day is a Saturday, Sunday or legal holiday under the
laws of the State in which the Property is located, in which event the period
shall run until the end of the next day which is neither a Saturday, Sunday or
legal holiday. The final day of any such period shall be deemed to end at 5
p.m., local time.

      12.10 Successors and Assigns. The terms and provisions of this Agreement
are to apply to and bind the permitted successors and assigns of the parties
hereto.

      12.11 Entire Agreement. This Agreement, including the Exhibits, contains
the entire agreement between the parties pertaining to the subject matter hereof
and fully supersedes all prior written or oral agreements and understandings
between the parties pertaining to such subject matter.

      12.12 Further Assurances. Each party agrees that it will without further
consideration execute and deliver such other documents and take such other
action, whether prior or subsequent to Closing, as may be reasonably requested
by the other party to consummate more effectively the purposes or subject matter
of this Agreement. Without limiting the generality of the foregoing, Cornerstone
shall, if requested by Contributors, execute acknowledgments of receipt with
respect to any materials delivered by Contributors to Cornerstone with respect
to the Property. The provisions of this Section 12.12 shall survive Closing.

      12.13 Counterparts. This Agreement may be executed in counterparts, and
all such executed counterparts shall constitute the same agreement. It shall be
necessary to account for only one such counterpart in proving this Agreement.

      12.14 Severability. If any provision of this Agreement is determined by a
court of competent jurisdiction to be invalid or unenforceable, the remainder of
this Agreement shall nonetheless remain in full force and effect.

      12.15 Applicable Law. This Agreement is performable in the state in which
the Property is located and shall in all respects be governed by, and construed
in accordance with, the substantive federal laws of the United States and the
laws of such state. Contributors and Cornerstone hereby irrevocably submit to
the jurisdiction of any state or federal court sitting in the state in which the
Property is located in any action or proceeding arising out of or relating to
this Agreement and hereby irrevocably agree that all claims in respect of such
action or proceeding shall be heard and determined in a state or federal court
sitting in the state in which the Property is located. Cornerstone and
Contributors agree that the provisions of this section 12.15 shall survive the
Closing of the transaction contemplated by this Agreement.

      12.16 No Third Party Beneficiary. The provisions of this Agreement and of
the documents to be executed and delivered at Closing are and will be for the
benefit of Contributors, Cornerstone and Operating Partnership only and are not
for the benefit of any third party, and accordingly, no third party shall have
the right to enforce the provisions of this Agreement or of the documents to be
executed and delivered at Closing.


                                       36
<PAGE>

      12.17 Exhibits and Schedules. The following schedules or exhibits attached
hereto shall be deemed to be an integral part of this Agreement:

            (a)   Exhibit A-1 -  Legal Description of the Phase I Land
            (b)   Exhibit A-2 -  Legal Description of the County Parcel
            (c)   Exhibit A-3 -  Legal Description of the Phase II Land
            (d)   Exhibit B-1 -  Phase I Personal Property
            (e)   Exhibit B-2 -  Phase II Personal Property
            (f)   Exhibit C-1 -  Phase I Lease Schedule
            (g)   Exhibit C-2 -  Phase II Lease Schedule
            (h)   Exhibit D   -  Operating Agreements Schedule
            (i)   Exhibit E   -  Tenant Estoppel Form
            (j)   Exhibit F   -  Partnership Agreement
            (k)   Exhibit G   -  Registration Rights Agreement
            (l)   Exhibit H-1 -  Tenant Inducement Costs Payable by Operating
                                 Partnership
            (m)   Exhibit H-2 -  Tenant Inducement Costs Payable by Contributors
            (n)   Exhibit I   -  Tenant Improvement Loans
            (o)   Exhibit J-1 -  Phase I Litigation
            (p)   Exhibit J-2 -  Phase II Litigation
            (q)   Exhibit K-1 -  Phase I Lease Matters
            (r)   Exhibit K-2 -  Phase II Lease Matters
            (s)   Exhibit L-1 -  Phase I Security Deposits
            (t)   Exhibit L-2 -  Phase II Security Deposits
            (u)   Exhibit M-1 -  Phase I Leasing Commissions
            (v)   Exhibit M-2 -  Phase II Leasing Commissions
            (w)   Exhibit N-1 -  Phase I Rent Roll
            (x)   Exhibit N-2 -  Phase II Rent Roll

      12.18 Captions. The section headings appearing in this Agreement are for
convenience of reference only and are not intended, to any extent and for any
purpose, to limit or define the text of any section or any subsection hereof.

      12.19 Construction. The parties acknowledge that the parties and their
counsel have reviewed and revised this Agreement and that the normal rule of
construction to the effect that any ambiguities are to be resolved against the
drafting party shall not be employed in the interpretation of this Agreement or
any exhibits or amendments hereto.

      12.20 Termination of Agreement. It is understood and agreed that if any of
Cornerstone or Contributors terminates this Agreement pursuant to a right of
termination granted hereunder, such termination shall operate to relieve
Contributors and Cornerstone from all obligations under this Agreement, except
for such obligations as are specifically stated herein to survive the
termination of this Agreement.


                                       37
<PAGE>

      12.21 Survival. The provisions of Article XI and the following Sections of
this Agreement shall survive Closing and shall not be merged into the execution
and delivery of the Deeds: 4.1; 5.2(j); 5.5; 6.5; 6.8; 9.1; 10.3; 12.1; 12.8;
12.12; and 12.15.


                                       38
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement
as of the Effective Date.


                         CONTRIBUTOR:

                         CORPORATE 500-PHASE I, an Illinois limited
                         partnership

                         By:   Matas Partners, an Illinois limited partnership,
                               its general partner

                               By:      /s/ Myron Levin
                                        -----------------------------
                               Name:    Myron Levin
                               Title:   General Partner


                         CORPORATE 500-PHASE I, an Illinois limited
                         partnership

                         By:   Matas Partners, an Illinois limited partnership,
                               its general partner

                               By:      /s/ Myron Levin
                                        -----------------------------
                               Name:    Myron Levin
                               Title:   General Partner

                         CORNERSTONE:

                         CORNERSTONE PROPERTIES, INC.,
                         a Nevada corporation

                               By:      /s/ (signature illegible)
                                        -----------------------------
                               Name:
                                        -----------------------------
                               Title:
                                        -----------------------------

                               By:      /s/ Francis H. Shields, Jr.
                                        -----------------------------
                               Name:    Francis H. Shields, Jr.
                               Title:   Vice President
<PAGE>

                                  Exhibit A-1

                     LEGAL DESCRIPTION OF THE PHASE I LAND

LOT 1 EXCEPT THE SOUTH 20 FEET THEREOF) IN CORPORATE 500 SUBDIVISION, A
SUBDIVISION OF PART OF THE SOUTHWEST 1/4 OF SECTION 33, TOWNSHIP 43 NORTH, RANGE
12, EAST OF THE THIRD PRINCIPAL MERIDIAN, ACCORDING TO THE PLAT THEREOF RECORDED
FEBRUARY 4, 1988 AS DOCUMENT 2654632, IN LAKE COUNTY, ILLINOIS.
<PAGE>

                                   Exhibit A-2

                     LEGAL DESCRIPTION OF THE COUNTY PARCEL

THE NORTH 217 FEET OF THE SOUTH 267 FEET OF THE WEST 276.18 FEET OF THE EAST
476.18 FEET OF THE SOUTH 788.62 FEET OF THE SOUTH HALF OF THE SOUTHWEST QUARTER
OF SECTION 33, TOWNSHIP 43 NORTH, RANGE 12 EAST OF THE THIRD PRINCIPAL MERIDIAN,
(EXCEPTING THEREFROM THE SOUTH 20 FEET THEREOF), IN LAKE COUNTY, ILLINOIS.
<PAGE>

                                   Exhibit A-3

                     LEGAL DESCRIPTION OF THE PHASE II LAND

PARCEL 1:

LOT 2 IN CORPORATE 500 SUBDIVISION, A SUBDIVISION OF PART OF THE SOUTH WEST 1/4
OF SECTION 33, TOWNSHIP 43 NORTH, RANGE 12, EAST OF THE THIRD PRINCIPAL
MERIDIAN, ACCORDING TO THE PLAT THEREOF RECORDED FEBRUARY 4, 1988 AS DOCUMENT
2654632, IN LAKE COUNTY, ILLINOIS.

PARCEL 2:

EASEMENT FOR PARKING AND INGRESS AND EGRESS FOR THE BENEFIT OF PARCEL 1 OVER THE
SOUTHERLY 15 FEET OF THE FOLLOWING DESCRIBED PROPERTY: THAT PART OF THE SOUTH
WEST 1/4 OF SECTION 33, TOWNSHIP 43 NORTH, RANGE 12, EAST OF THE THIRD PRINCIPAL
MERIDIAN, DESCRIBED AS FOLLOWS: COMMENCING AT THE SOUTH EAST CORNER OF SAID
SOUTH WEST 1/4; THENCE NORTH 90 DEGREES 00 MINUTES 00 SECONDS WEST, A DISTANCE
OF 876.18 FEET ALONG THE SOUTH LINE OF SAID SOUTH WEST 1/4 TO THE NORTHEASTERLY
LINE OF THE CHICAGO, MILWAUKEE, ST. PAUL AND PACIFIC RAILROAD; THENCE NORTH 25
DEGREES 09 MINUTES 46 SECONDS WEST, A DISTANCE OF 1,944.80 FEET ALONG THE
NORTHEASTERLY LINE OF SAID RAILROAD TO A POINT ON THE SOUTHEASTERLY LINE OF
KATES ROAD; THENCE NORTH 32 DEGREES 28 MINUTES 02 SECONDS EAST, A DISTANCE OF
69.23 FEET ALONG THE SOUTHEASTERLY LINE OF SAID ROAD TO THE POINT OF BEGINNING;
THENCE NORTH 32 DEGREES 28 MINUTES 02 SECONDS EAST A DISTANCE OF 185.48 FEET TO
A POINT ON THE SOUTH LINE OF SAID KATES ROAD; THENCE SOUTH 89 DEGREES 54 MINUTES
13 SECONDS EAST ALONG SAID SOUTH LINE, A DISTANCE OF 824.12 FEET TO A POINT;
THENCE SOUTHWESTERLY ALONG THE ARC OF A CIRCLE HAVING A RADIUS OF 2,251.83 FEET
AND CONVEX NORTHWESTERLY, A DISTANCE OF 215.94 FEET TO A POINT; THENCE SOUTH 84
DEGREES 36 MINUTES 14 SECONDS WEST ALONG TANGENT, A DISTANCE OF 392.01 FEET TO A
POINT; THENCE SOUTHWESTERLY ALONG THE ARC OF A CIRCLE HAVING A RADIUS OF 634.07
FEET AND CONVEX NORTHWESTERLY, A DISTANCE OF 219.82 FEET TO A POINT; THENCE
SOUTH 64 DEGREES 44 MINUTES 24 SECONDS WEST, A DISTANCE OF 118.14 FEET TO THE
POINT OF BEGINNING, AS GRANTED BY INSTRUMENT RECORDED DECEMBER 8, 1988 AS
DOCUMENT 2747296, (EXCEPT THAT PART THEREOF LYING EAST OF THE WEST LINE OF
PARCEL 2 EXTENDED NORTHERLY) IN LAKE COUNTY, ILLINOIS.
<PAGE>

                              Exhibits B-1 and B-2


                    LIST OF PERSONAL PROPERTY OF THE COMPANY

                  [To be provided upon request to the Company.]
<PAGE>

                                   Exhibit C-1

                             PHASE I LEASE SCHEDULE
                             As of December 17, 1997


                  [To be provided upon request to the Company.]
<PAGE>

                                   Exhibit C-2


                             PHASE II LEASE SCHEDULE
                             As of December 17, 1997


                  [To be provided upon request to the Company.]
<PAGE>

                                    Exhibit D

                                   C500 CENTRE
                          OPERATING AGREEMENTS SCHEDULE

                             As of December 17, 1997

-------------------------------------------------------------------------------
        VENDOR NAME                  SERVICE                      EXPIRATION
-------------------------------------------------------------------------------
B.P. Services                      Snow plowing                     4/30/98
Brickman Landscaping               Landscaping                     11/30/98
Champion recycling                 Recycling                    30 day's notice
Dover Elevator                     Phase II elevator              90 days (?)
Kroschell Engineering Co.          HVAC                         30 days notice
Lakeside Building Maintenance      Janitorial                       At will
Smith Security                     Security guard               5 days' notice
U.S. Elevator                      Phase I elevator                 90 days
North Shore Waste Control          Scavenger                        7/1/00
Up 'N Adam Service & Supply Inc.   Restaurant equip. maint.        Any time
Hobart                             Dishwasher                       5/31/98
Alliance Refrigeration             Restaurant refrigeration     30 days notice
ATT Automotive Services            Auto Lease                      12/31/98
<PAGE>

                                    Exhibit E

                              TENANT ESTOPPEL FORM

Lease Date:

Landlord:       American National Bank and Trust Company of Chicago, not
                personally, but as Trustee under Trust No. 59087 [for Phase I,
                or 65110 for Phase II]

Tenant:

Premises:       Corporate 500 Centre, Building No. _______, _______ Floor

Area:           _________________________________, Sq. Ft.

The undersigned Tenant of the above-referenced lease (the "Lease") hereby
ratifies the Lease and certifies to Landlord and Cornerstone Deerfield LLC
("Purchaser") as purchaser of the Real Property of which the premises demised
under the Lease (the "Premises") is a part, as follows:

1.    That the term of the Lease commenced on __________________, 19__ and the
      Tenant is in full complete possession of the Premises and has commenced
      full occupancy and use of the Premises, such possession having been
      delivered by the original landlord and having been accepted by the Tenant
      or if the term has not commenced that it will commence on _______________,
      19__ and Tenant will take occupancy on _______________, 19__.

2.    That the Lease calls for the current payment of monthly installments of
      Base Rent of $__________. [Abatement of Base Rent under the Lease runs
      through__________, 19__ and thereafter the Tenant is paying monthly
      installments of Base Rent of $_________ which commence on the ___ day of
      ___________, 19__.]

3.    That no advance rental or other payment has been made in connection with
      the Lease, except rental for the current month, and the rent has been paid
      to and including ____________, 19__.

4.    That a security deposit in the amount of $___________ is being held by
      Landlord, which amount is not subject to any set-off or reduction or to
      any increase for interest or other credit due to Tenant.

5.    That all obligations and conditions under said Lease to be performed to
      date by Landlord or Tenant have been satisfied, free of defenses and
      set-offs including all construction work in the Premises.


                                      E-1
<PAGE>

6.    That the Lease is a valid lease and in full force and effect and
      represents the entire agreement between the parties; that there is no
      existing default on the part of the Landlord or the Tenant in any of the
      terms and conditions thereof and no event has occurred which, with the
      passing of time or giving of notice or both, would constitute an event of
      default; and that said Lease has not been amended, modified, supplemented,
      extended, renewed, subleased or assigned, except as follows by the
      following described agreements:

7.    That the Lease provides for a primary term of ____________ months; the
      term of the Lease expires on the day of _______________, 19__; and that
      neither the Lease nor any of the documents listed in Paragraph 6 (if any),
      contain an option for any additional term or term [or the Lease and/or the
      documents listed under Paragraph 6, above, contain an option for
      __________________ additional term(s) of _________ year(s) and ________
      month(s) (each) at a rent to be determined as set forth in the Lease].

8.    That there are no expansion rights or rights of first refusal or first
      offer to lease additional space except as set forth in the Lease and/or
      the documents listed under Paragraph 6 above.

9.    That there are no actions, voluntary or involuntary, pending against the
      Tenant under the bankruptcy laws of the United States or any state
      thereof.

10.   That this certification is made knowing that Landlord, Purchaser and
      Purchaser's lender are relying upon the representations herein made.

Dated _________________, 19__.

                                          TENANT:


                                          -----------------------------------

                                          By: _______________________________
                                          Name: _____________________________
                                          Title: ____________________________


                                      E-2
<PAGE>

                                    Exhibit F


                        AGREEMENT OF LIMITED PARTNERSHIP
                                       OF
                   CORNERSTONE PROPERTIES LIMITED PARTNERSHIP


        [See Exhibit 10.118 to the Company's Annual Report filed on Form
                   10-K for the year ended December 31, 1997]
<PAGE>

                                    Exhibit G


                          REGISTRATION RIGHTS AGREEMENT


                  [To be provided upon request to the Company.]
<PAGE>

                                   Exhibit H-1

            TENANT INDUCEMENT COSTS PAYABLE BY OPERATING PARTNERSHIP


1.    Specialty Foods Corporation (improvement allowance)       $100,920

2.    Knowledge University (leasing commissions)                  40,512
<PAGE>

                                   Exhibit H-2

                 TENANT INDUCEMENT COSTS PAYABLE BY CONTRIBUTORS

1.    MMI Companies, Inc. (gross improvement
            allowance of $311,880 less $54,994
            costs paid to date)                                  $256,886

2.    Prudential Securities Inc. (improvement
            allowance)                                            200,000
<PAGE>

                                    Exhibit I

                            TENANT IMPROVEMENT LOANS


Franklin Enterprises                                             $ 14,656

Drake Beam Morin                                                  102,619
<PAGE>

                                   Exhibit J-1

                               PHASE I LITIGATION


Kleinschmidt, Inc. v. The County of Cook, et al.,
No. 95 CH 501 (Circuit Court of Lake County, Illinois)

      On or about June 20, 1995, this lawsuit was filed by Plaintiff,
Kleinschmidt, Inc., against defendants Cook County, Lake County, Corporate 500
Centre Phase I, and American National Bank and Trust Company of Chicago, as
trustee under Trust No. 59087 (the "Trust"). Plaintiff's complaint sought
injunctive and permanent relief from defendants for allegedly violating
Plaintiff's constitutional rights by Cook County's conveyance of Plaintiff's
former property to the Trust.

      On or about May 30, 1996, the trial court entered summary judgment in
favor of all defendants and against Plaintiff on all counts of its Complaint. On
March 19, 1997, the Illinois Appellate Court for the Second District affirmed
the trial court's entry of summary judgment. The decision is published as
Kleinschmidt v. County of Cook, 678 N.E.2d 1065 (2nd Dist. 1997). On or about
October 1, 1997, the Illinois Supreme Court defined Plaintiff's petition for
leave to appeal the Appellate Court decision to the Illinois Supreme Court.
<PAGE>

                                   Exhibit J-2

                               PHASE II LITIGATION


                                      None
<PAGE>

                                   Exhibit K-1

                              PHASE I LEASE MATTERS


                                      None
<PAGE>

                                   Exhibit K-2

                             PHASE II LEASE MATTERS

                                      None
<PAGE>

                                   Exhibit L-1

                                    PHASE I
                                SECURITY DEPOSITS


             TENANT                               AMOUNT
--------------------------------------------------------------------------------
      OCCONNOR                                         1,488.00
      TRAVEL DUET                                      4,555.63
      TRAVEL DUET                                      1,293.46
      AMER BOARD                                       9,747.96
      DIGINET                                          1,400.00
                                           ------------------------
                                                      18,485.05
                                           ========================


      KNOWLEDGE UNIVERSITY (LETTER OF CREDIT)        500,000.00
<PAGE>

                                   Exhibit L-2

                           PHASE II SECURITY DEPOSITS

                                                   SECURITY
                     TENANT                         DEPOSIT
--------------------------------------------------------------------------------
           FRANKLIN                                   3,100.50
           JACQUARD                                   3,126.00
           DAVID GILFAND                              4,357.33
           DRAKE BEAM                                10,087.00
           RANK VIDEO                                54,829.42
           MCG ELLERMAN                              17,983.59
           ELKINS                                     2,253.75
           XMEN                                       1,590.00
           ARBITRATION FORUMS                         4,248.63
           FIRST PREMIUM                              8,300.00
           AMERICAN AIAKOKU                           3,466.67
           CITI PLATE                                 2,641.17
           INTREPID                                   4,625.50
                                            ----------------------

           TOTAL                                    120,609.56
                                            ======================
           NEW YORK LIFE
           (CONSTRUCTION DEPOSIT)                  $211,233.00
<PAGE>

                                   Exhibit M-1

                           PHASE I LEASING COMMISSIONS

Knowledge University - $58,887 commission to Syndicated Equities Inc., $18,375
of which was paid by prior tenant (Acco). The balance of $40,512 is payable by
the Operating Partnership (see Exhibit H-1).
<PAGE>

                                   Exhibit M-2

                          PHASE II LEASING COMMISSIONS


New York Life (pending amendment) - $11,960 commission to Grubb & Ellis to be
paid by the Operating Partnership.
<PAGE>

                                   Exhibit N-1

                                  C500 PHASE I
                              COMMERCIAL RENT ROLL
                         REPORT DATE: 1/1/98 TO 1/31/98
                                 as of 12/17/97


                 [To be provided upon request to the Company.]
<PAGE>

                                  Exhibit N-2

                                 C500 PHASE II
                              COMMERCIAL RENT ROLL
                         REPORT DATE: 1/1/98 TO 1/31/98
                                 as of 12/17/98


                 [To be provided upon request to the Company.]