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                                                                    Exhibit 10.2


                                VOTING AGREEMENT

                                     (PGGM)


                  THIS VOTING AGREEMENT (this "AGREEMENT") is entered into as of
February 11, 2000 by and among Equity Office Properties Trust, a Maryland real
estate investment trust ("EOP"), EOP Operating Limited Partnership, a Delaware
limited partnership ("EOP PARTNERSHIP"), WCP Services, Inc., a Delaware
corporation ("WCP"), and Stichting Pensioenfonds voor de Gezondheid, Geestelijke
en Maatschappelijke Belangen, a stichting formed according to the laws of the
Kingdom of The Netherlands ("PGGM").

                  WHEREAS, EOP, EOP Partnership, Cornerstone Properties, Inc., a
Nevada corporation ("Cornerstone"), Cornerstone Properties Limited Partnership,
a Delaware limited partnership ("CORNERSTONE PARTNERSHIP"), will enter into an
Agreement and Plan of Merger dated as of the date hereof (the "MERGER
Agreement"), pursuant to which (i) Cornerstone Partnership will be merged with
and into EOP Partnership (the "PARTNERSHIP MERGER"), with EOP Partnership as the
survivor of the Partnership Merger, and (ii) Cornerstone will be merged with and
into EOP (the "MERGER"), with EOP as the survivor of the Merger (all capitalized
terms used but not defined herein shall have the meanings set forth in the
Merger Agreement);

                  WHEREAS, PGGM is the beneficial and record owner of 45,779,703
issued and outstanding shares of common stock, with no par value per share, of
Cornerstone (such shares, together with any shares acquired hereafter, the
"CORNERSTONE COMMON SHARES") as more particularly described on SCHEDULE 1;

                  WHEREAS, in accordance with the Recitals in the Merger
Agreement, PGGM desires to execute and deliver this Agreement solely in its
capacity as a holder of Cornerstone Common Shares; and

                  WHEREAS, to induce PGGM to enter into this Agreement, EOP, EOP
Partnership and WCP desire to make certain undertakings and agreements as set
forth herein.

                  NOW, THEREFORE, in consideration of the foregoing and for
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

SECTION 1.        DISPOSITION OF CORNERSTONE COMMON SHARES

                  During the period from the date hereof through the earlier of
(i) the date on which the Merger is consummated or (ii) 30 days after the date
on which

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the Merger Agreement is terminated according to its terms (such period
hereinafter referred to as the "TERM"), PGGM shall not, directly or indirectly,
(a) sell, transfer, pledge, encumber, assign or otherwise dispose of, or enter
into any contract, option or other agreement or understanding with respect to
the sale, transfer, pledge, encumbrance, assignment or other disposition of, any
Cornerstone Common Shares, (b) grant any proxies for any Cornerstone Common
Shares with respect to any matters described in Section 2(a) hereof (other than
a proxy directing the holder thereof to vote the Cornerstone Common Shares in a
manner required by Section 2(a) hereof), (c) deposit any Cornerstone Common
Shares into a voting trust or enter into a voting agreement with respect to any
Cornerstone Common Shares with respect to any matters described in Section 2(a)
hereof, or tender any Cornerstone Common Shares in a transaction other than a
transaction contemplated by the Merger Agreement, or (d) take any action which
is intended to have the effect of preventing or disabling PGGM from performing
its obligations under this Agreement; PROVIDED, HOWEVER, that nothing herein
shall prevent the sale, transfer, pledge, encumbrance, assignment or other
disposition of any of such Cornerstone Common Shares, provided that the
purchaser, transferee, pledgee or assignee thereof agrees in writing to be bound
by the terms of this Agreement.

SECTION 2.        VOTING

                  (a) During the Term, PGGM shall cast or cause to be cast all
votes attributable to the Cornerstone Common Shares, at any annual or special
meeting of shareholders of Cornerstone, including any adjournments or
postponements thereof, or in connection with any written consent or other vote
of Cornerstone shareholders, (i) in favor of adoption of the Merger Agreement
and approval of the Merger and the other transactions contemplated by the Merger
Agreement (including any amendments or modifications of the terms of the Merger
Agreement approved by the board of directors of Cornerstone that would not
materially adversely affect PGGM in its capacity as beneficial owner of
Cornerstone Common Stock), and (ii) against approval or adoption of any action
or agreement (other than the Merger Agreement or the transactions contemplated
thereby) made or taken in opposition to or in competition with the Merger or the
Partnership Merger.

                  (b) PGGM will retain the right to vote its Cornerstone Common
Shares, in its sole discretion, on all matters other than those described in
paragraph (a) of this Section 2, and PGGM may grant proxies and enter into
voting agreements or voting trusts for its Cornerstone Common Shares in respect
of such other matters.

SECTION 3.        NON-SOLICITATION

                  During the Term, PGGM (a) shall not, and shall not permit any
of its officers, directors, employees, Affiliates, agents, investment bankers,
financial advisors, attorneys, accountants, brokers, finders or other
representatives retained

                                      -2-
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by it to, (i) invite, initiate, solicit or encourage, directly or indirectly,
any inquiries, proposals, discussions or negotiations or the making or
implementation of any Acquisition Proposal, or (ii) engage in any discussions or
negotiations with or provide any confidential or non-public information or data
to, any person relating to an Acquisition Proposal, or otherwise facilitate any
effort or attempt to make or implement an Acquisition Proposal; and (b) shall
notify EOP immediately if it receives any such inquiries or proposals, or any
requests for such information, or if any such negotiations or discussions are
sought to be initiated or continued with PGGM.

SECTION 4.        TAX MATTERS

                  (a) So long as there is no change in Section 1445 of the
Internal Revenue Code of 1986, as amended (the "Code"), the Treasury Regulations
promulgated thereunder, or the published interpretations of the Internal Revenue
Service with respect thereto occurring after the date hereof (a "Change in
Law"), EOP shall not withhold, and shall not cause to be withheld, any tax
pursuant to Section 1445 of the Code in respect of the Merger Consideration to
be paid to PGGM pursuant to the Merger. In the event that EOP believes that a
Change in Law has occurred, it shall deliver written notice thereof to PGGM. In
the event that PGGM shall deliver to EOP an opinion of nationally recognized tax
counsel reasonably satisfactory to EOP to the effect that, taking into account
the Change in Law, EOP is not required to withhold any amount of federal tax
with respect to any portion of the Merger Consideration payable to PGGM under
the Merger Agreement, then the obligation of EOP set forth in the first sentence
of this subparagraph (a) shall continue in effect.

                  (b) (i) EOP shall designate distributions paid by EOP to its
shareholders that have a record date during 2000 prior to the Closing Date as
"capital gain dividends" (as defined in Section 857(b)((3)(C)) of the Code) in
an amount equal to the lesser of (A) the distributions paid by EOP to its
shareholders that have a record date during 2000 prior to the Closing Date
(reduced by any amount designated by EOP pursuant to Section 858(a) of the Code
and the Treasury Regulations thereunder as being paid during 1999), or (B) the
amount of gain that is recognized by EOP during the period commencing on January
1, 2000 and ending on the date prior to the Closing Date with respect to the
disposition of "United States real property interests" (as defined for purposes
of Section 897 of the Code) and that is otherwise eligible for designation as a
"capital gain dividend" under Section 857(b)(3)(C) of the Code, and (ii) it
shall cause the Form 1099s to be delivered to PGGM and the other EOP
shareholders with respect to the year 2000 to be prepared in a manner consistent
with the foregoing designation; PROVIDED, HOWEVER, that EOP makes no
representation or warranty to PGGM that the aforesaid designation will be
respected for federal income tax purposes and that EOP in no event shall have
any liability to PGGM by reason of a recharacterization by the Internal Revenue
Service of distributions paid by EOP to PGGM during or

                                      -3-
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with respect to 2000 as "capital gain dividends" or as otherwise including
income attributable to the disposition of "United States real property
interests." EOP also agrees not to withhold from amounts otherwise distributable
to PGGM any tax pursuant to Section 1445 attributable to "capital gains
dividends" (within the meaning of Section 857(b)(3)(C) of the Code) distributed
by EOP to its shareholders in 1999.

                  (c) Upon request from PGGM made from time to time (but not
more frequently than once each calendar quarter), EOP shall endeavor to deliver
to PGGM within fifteen (15) business days after the request therefor a statement
(based upon reasonable inquiry) to the effect that, to the knowledge of EOP, EOP
qualifies as a "domestically controlled REIT" (within the meaning of Section
897(h)(4)(B) of the Code) if such statement in fact would be true when made. For
purposes of such statement, reasonable inquiry shall include review of all
Schedule 13D and 13G filings made under the Exchange Act with the SEC with
respect to EOP during the lesser of the five calendar years preceding the date
of the statement or the period commencing July 1, 1997, all IRS Form 1042
filings made by or on behalf of EOP with respect to each of the five taxable
years preceding the date of the statement (or if shorter, the period commencing
July 1, 1997), the list of EOP's registered shareholders as of a date within 60
days of such statement (and to the extent reasonably available, as of a date
within 60 days of the end of each of the preceding five calendar years (or if
shorter, each of the calendar years commencing with 1997)), a report obtained by
EOP from a shareholders tracking service within 60 days of such statement (and
any similar reports in the possession of EOP or otherwise reasonably available
to EOP providing information as of a date within 60 days of the end of each of
the five preceding calendar years (or if shorter, each of the calendar years
commencing with 1997)), and a list of "non-objecting beneficial owners" of
shares of EOP obtained as of a date within 60 days of such statement (and to the
extent reasonably available, as of a date within 60 days of the end of each of
the preceding five calendar years (or if shorter, each of the calendar years
commencing with 1997)). Such statement shall be accompanied by copies of the
information that has been obtained or relied upon by EOP for purposes of such
statement, PROVIDED THAT PGGM shall have executed an agreement with EOP to treat
such information as confidential and to use such information solely for the
purposes of evaluating the accuracy of such statement. In the event that EOP
should determine in good faith that it cannot provide to PGGM the requested
statement for any reason, EOP shall notify PGGM of such conclusion and the facts
that cause it to be unable to render such statement. In addition to, and without
limiting, the foregoing, in the event that the General Counsel of EOP shall have
actual knowledge that more than 40 percent, by fair market value, of the
outstanding equity interests of EOP are owned directly or indirectly by "foreign
persons" (as that term is used for purposes of Section 897(h)(4)(B) of the
Code), EOP shall provide written notice thereof (together with a summary of the
relevant facts) to PGGM, PROVIDED THAT the only duty of inquiry of EOP shall be
as set forth in the first sentence of this subparagraph (c).

                                      -4-
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                  (d) In the event that EOP shall make any distributions to PGGM
that it concludes in good faith would be subject to withholding of tax pursuant
to the last sentence of Section 1445(e)(3) of the Code and any Treasury
Regulations promulgated with respect thereto, EOP shall provide such reasonable
cooperation as PGGM may request in applying to the Internal Revenue Service for
a "withholding certificate" that would reduce or eliminate the requirement for
such withholding; PROVIDED, HOWEVER, that PGGM shall be responsible for the
preparation and submission of the application for such withholding certificate
and that EOP shall not be precluded from withholding such tax unless and until a
"withholding certificate" is obtained (in which event EOP would not withhold tax
that, under the express terms of the "withholding certificate," is not required
to be withheld). In addition, in the event that PGGM shall provide to EOP an
opinion of nationally recognized tax counsel reasonably satisfactory to EOP to
the effect that EOP is not required pursuant to Section 1445(e)(3) of the Code
and any Treasury Regulations promulgated with respect thereto to withhold any
amount of federal tax with respect to any portion a distribution to PGGM, EOP
shall not withhold any such tax unless it shall conclude in good faith that a
Change in Law has occurred after the date of such opinion, in which event EOP
shall provide written notice thereof to PGGM. Thereafter, the preceding sentence
would not apply unless PGGM delivers an opinion of nationally recognized tax
counsel reasonably satisfactory to EOP reconfirming the original opinion, after
taking into account the Change in Law.

                  (e) The obligations of EOP set forth in subparagraphs (c) and
(d) shall terminate at such time as PGGM owns less than the lesser of (i) one
percent (1%) of the issued and outstanding EOP Common Shares or (ii) the number
of EOP Common Shares issued to PGGM in the Merger.

                  (f) PGGM agrees that, effective as of the Effective Time of
the Merger, all agreements and undertakings previously entered into by
Cornerstone or any Cornerstone Subsidiary with respect to tax matters,
including, without limitation, agreements restricting the sale or other
disposition of one or more assets owned by Cornerstone, Cornerstone Partnership,
or any Subsidiary of either Cornerstone or Cornerstone Partnership shall
terminate for events or transactions occurring after the Effective Time of the
Merger, and that neither EOP, EOP Partnership nor any Subsidiaries of either EOP
or EOP Partnership shall have any obligation or liability thereunder for events
or transactions occurring following the Effective Time of the Merger. Without
limiting the foregoing, such termination shall include the agreements of
Cornerstone and Cornerstone Partnership set forth in the letter dated June 22,
1998 from Cornerstone and Cornerstone Partnership to PGGM, the undertakings with
respect to tax matters set forth in the Amended and Restated Registration Rights
and Voting Agreement dated as of December 16, 1998 by and among Cornerstone,
PGGM, and Dutch Institutional Holding Company, Inc. (THE "REGISTRATION RIGHTS
AGREEMENT"), and the policy of Cornerstone with

                                      -5-
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respect to One Norwest Center, Denver, Colorado (and any properties acquired in
exchange therefor) adopted by its Board of Directors at a meeting on August 13,
1997.

SECTION 5.        CONSENT TO TRANSFER

                  To the extent required by any mortgage, pledge, security
agreement, deed of trust or other agreement or instrument entered into by
Cornerstone or any of its Affiliates with or for the benefit of PGGM, including,
without limitation, the agreements listed in item 5 on EXHIBIT A attached hereto
(collectively, the "INSTRUMENTS"), PGGM agrees and acknowledges that, subject to
and effective as of the Effective Time of the Merger, without any further action
by Cornerstone, EOP, PGGM or any other party thereto or beneficiary thereof,
PGGM hereby consents to the transfer to EOP and its Affiliates as a result of
the Mergers and other transactions contemplated by the Merger Agreement of the
beneficial ownership interest of any of the Affiliates of Cornerstone who are a
party to the Instruments and to each other matter thereunder with respect to
which PGGM's consent is required in connection with the Mergers and other
transactions contemplated by the Merger Agreement.

SECTION 6.        REGISTRATION RIGHTS AGREEMENT

                  PGGM and EOP hereby agree and acknowledge that, subject to and
effective as of the Effective Time of the Merger, without any further action by
Cornerstone, EOP or PGGM, EOP and PGGM shall be bound by the Registration Rights
Agreement as the same is amended as follows:

                  (a) From and after the Effective Time of the Merger, (i) all
references in the Registration Rights Agreement to the "Company" shall be deemed
to be references to EOP; (ii) all references therein to DIHC shall be deleted;
(iii) the references to "that together own 25% or more of the issued and
outstanding Common Stock" in Sections 1.2(iii) and 1.2(iv) shall be deleted;
(iv) all references therein to "Common Stock" shall refer to the EOP Common
Shares, (v) the "Initial Percentage" shall mean 12% of the issued and
outstanding Common Stock; (vi) the "Standstill Period" shall mean, with respect
to any Holder, a period of time commencing on the Effective Time of the Merger
and terminating ninety (90) days after the first date following the election of
PGGM's designees to EOP's Board of Trustees that no Trustee designated by PGGM
pursuant to Section 7 of this Voting Agreement remains a Trustee of EOP; and
(vii) all references therein to "Units" shall refer to EOP OP Units.

                  (b) Section 2 shall be deleted in its entirety and all
references to Section 2 in the Registration Rights Agreement shall be deleted.

                                      -6-
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                  (c) Section 3.1(a) shall be amended to delete the reference to
the specified Cornerstone Registration Statement and to cause the first line to
read as follows: "Within 20 days after the request of PGGM following the
Effective Time of the Merger."

                  (d) Section 7 shall be deleted in its entirety and all
references to Section 7 in the Registration Rights Agreement shall be deleted.

                  (e) Section 8 shall be amended as follows: (i) the reference
to "that together with its Affiliates owns 25% or more of the issued and
outstanding shares of Common Stock" in Section 8 shall be deleted, and (ii) the
reference to "Section 78.140 of the Nevada General Corporation Law" shall refer
to Section 2-419 of the Maryland General Corporation Law.

                  (f) Section 9 shall be amended as follows: (i) deleting "and"
after the end of clause (I), (ii) adding "and" after the end of clause (II), and
(iii) adding a new clause (III) as follows: "(III) by the Company in connection
with a Business Combination to which the Company is a party."

                  (g) Section 11 shall be amended to refer to the address of EOP
set forth in the Merger Agreement.

                  (h) Sections 12 and 13 shall be deleted.

SECTION 7.        TRUSTEES

                  The trustees of EOP following the Merger shall consist of the
trustees of EOP immediately prior to the Effective Time of the Merger, who shall
continue to serve for the balance of their unexpired terms or their earlier
death, resignation or removal, together with John S. Moody, William Wilson III
and Jan van der Vlist, each of whom shall, no later than the third business day
after the Effective Time of the Merger, become a trustee with terms expiring in
2002, 2003 and 2003, respectively. Upon the expiration of the terms of Mr. van
der Vlist in 2003 and 2006, so long as PGGM and its Affiliates continue to own
in the aggregate 21,000,000 (as adjusted for stock splits, reverse stock splits,

                                      -7-
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stock dividends and similar actions) or more of the issued and outstanding EOP
Common Shares at all times up to the meeting of shareholders at which trustees
are being elected in such years, EOP shall take all action necessary to nominate
Mr. van der Vlist for re-election as a trustee of EOP for an additional
three-year term at any special or annual meeting of shareholders at which
trustees are being elected (or in connection with a written consent in lieu of a
meeting pursuant to which trustees are proposed to be elected). In the event
that Mr. Van der Vlist shall fail to stand for re-election as aforesaid for any
reason in either 2003 or 2006 or in the event of his earlier death or
resignation, and so long as PGGM and its Affiliates continue to own in the
aggregate 21,000,000 (as adjusted for stock splits, reverse stock splits, stock
dividends and similar actions) or more of the issued and outstanding EOP Common
Shares at such time, EOP shall take all action necessary to nominate a
replacement designated by PGGM, which replacement shall be subject to the
approval of EOP if such replacement is not an officer, director or employee of
PGGM, for election or re-election as a trustee of EOP for an additional
three-year term at any special or annual meeting of shareholders at which
trustees are being elected (or in connection with a written consent in lieu of a
meeting pursuant to which trustees are proposed to be elected) or, in the case
of a vacancy, at a meeting of the Board of Trustees called for such purpose.
Except as expressly provided above in this Section 7, following their election
as trustees, such persons shall serve for their designated terms, subject to
their earlier death, resignation or removal.

SECTION 8.        CERTAIN AGREEMENTS

                  PGGM represents and warrants to EOP, EOP Partnership and WCP
that (a) the only pending claims asserted against PGGM or Robert T. Sorrentino,
Craig W. Johnston or Barrington H. Branch (the "Individuals") under the
Indemnity Agreement, dated as of October 27, 1997, as thereafter amended, among
DIHC Holding Company, Inc., PGGM and Cornerstone (the "Indemnity Agreement") or
any of the agreements listed in items 1, 2, 3 and 4 on EXHIBIT A attached hereto
(collectively, the "Purchase Agreements") are (i) the Western Litigation (as
defined in the Indemnity Agreement) and (ii) the Massachusetts state tax claim
previously disclosed to EOP in writing (the "Massachusetts Tax Claim"), and (b)
to its knowledge after reasonable inquiry, there is no basis for any further
claim, obligation, or liability of PGGM or any of the Individuals under the
Indemnity Agreement or any of the Purchase Agreements. EOP, EOP Partnership and
WCP hereby agree and acknowledge that, subject to and effective as of the
Effective Time of the Merger, without any further action by EOP, EOP
Partnership, WCP or PGGM, PGGM and the Individuals shall be released and
discharged from any and all claims, obligations or liabilities under the
Indemnity Agreement and the Purchase Agreements, including, without limitation,
with respect to or in connection with the Massachusetts Tax Claim (collectively,
the "Released Claims"), EXCEPT for (x) any claims, obligations or liabilities
with respect to or in connection with the Western Litigation and (y) any claims,
obligations or liabilities a basis for which PGGM has or would have had
knowledge after reasonable inquiry as of the Effective Time (collectively (x)
and (y) being referred to as the "Excluded Claims"). From and after the
Effective Time, EOP and EOP Partnership shall indemnify, defend and hold
harmless PGGM from and against any and all cost, claim, liability, damage or
expense (including, without limitation, reasonable attorneys' fees) with respect
to or in connection with the Released Claims and PGGM and the Individuals shall
be obligated under the Indemnity Agreement and the Purchase Agreements to EOP
and EOP Partnership with respect to or in connection with the Excluded Claims.

                                      -8-
<PAGE>

SECTION 9.        REPRESENTATIONS AND WARRANTIES OF PGGM

                  PGGM represents and warrants to EOP and EOP Partnership as
follows:

                  (a) PGGM has the legal capacity, power, authority and right
(contractual or otherwise) to execute and deliver this Agreement and to perform
its obligations hereunder. PGGM has obtained all consents of third parties
necessary to enter into this Agreement and to perform its obligations hereunder,
including, without limitation, the amendments to the Registration Rights
Agreement.

                  (b) This Agreement has been duly executed and delivered by
PGGM and constitutes a valid and binding obligation of PGGM enforceable against
PGGM in accordance with its terms, subject to applicable bankruptcy, insolvency,
moratorium or other similar laws relating to creditors rights and general
principles of equity.

                  (c) The execution and delivery of this Agreement and the
consummation of the transactions herein contemplated will not conflict with or
violate any court order, judgment or decree applicable to PGGM, or conflict with
or result in any breach of or constitute a default (or an event which with
notice or lapse of time or both would become a default) under any contract or
agreement to which PGGM is a party or by which PGGM is bound or affected, which
conflict, violation, breach or default would materially and adversely affect
PGGM's ability to perform any of its obligations under this Agreement.

                  (d) Subject to any required filings under the Securities
Exchange Act of 1934 (the " '34 Act"), PGGM is not required to give any notice
or make any report or other filing with any governmental authority in connection
with the execution or delivery of this Agreement or the performance of PGGM's
obligations hereunder and no waiver, consent, approval or authorization of any
governmental or regulatory authority or any other person or entity is required
to be obtained by PGGM for the performance of PGGM's obligations hereunder,
other than where the failure to make such filings, give such notices or obtain
such waivers, consents, approvals or authorizations would not materially and
adversely affect PGGM's ability to perform this Agreement.

                  (e) Cornerstone Common Shares set forth opposite PGGM on
SCHEDULE 1 hereto are the only Cornerstone Common Shares or other Cornerstone or
Cornerstone Partnership securities owned beneficially or of record by PGGM or
over which it exercises voting control.

                                      -9-
<PAGE>

SECTION 10.       REPRESENTATIONS AND WARRANTIES OF EOP AND EOP PARTNERSHIP

                  EOP and EOP Partnership represent and warrant to PGGM as
follows:

                  (a) Each of EOP and EOP Partnership has the legal capacity,
power, authority and right (contractual or otherwise) to execute and deliver
this Agreement and to perform its obligations hereunder. Each of EOP and EOP
Partnership has obtained all consents of third parties necessary to enter into
this Agreement and to perform its obligations hereunder.

                  (b) This Agreement has been duly executed and delivered by EOP
and EOP Partnership and constitutes a valid and binding obligation of EOP and
EOP Partnership enforceable against them in accordance with its terms, subject
to applicable bankruptcy, insolvency, moratorium or other similar laws relating
to creditors rights and general principles of equity.

                  (c) The execution and delivery of this Agreement and the
consummation of the transactions herein contemplated will not conflict with or
violate any court order, judgment or decree applicable to EOP or EOP
Partnership, or conflict with or result in any breach of or constitute a default
(or an event which with notice or lapse of time or both would become a default)
under any contract or agreement to which EOP or EOP Partnership is a party or by
which EOP or EOP Partnership is bound or affected, which conflict, violation,
breach or default would materially and adversely affect EOP or EOP Partnership's
ability to perform any of its obligations under this Agreement.

                  (d) Subject to any required filings under the Securities
Exchange Act of 1934 (the " '34 Act"), neither EOP nor EOP Partnership is
required to give any notice or make any report or other filing with any
governmental authority in connection with the execution or delivery of this
Agreement or the performance of its obligations hereunder and no waiver,
consent, approval or authorization of any governmental or regulatory authority
or any other person or entity is required to be obtained by EOP or EOP
Partnership for the performance of its obligations hereunder, other than where
the failure to make such filings, give such notices or obtain such waivers,
consents, approvals or authorizations would not materially and adversely affect
its ability to perform this Agreement.

SECTION 11.       FURTHER ASSURANCES

                  PGGM shall make such filings as may be required from time to
time under the '34 Act. The parties hereto shall, upon request by the other, do,
execute, acknowledge, deliver, record, re-record, file, re-file, register and
re-register any and all such further acts, deeds, conveyances, security
agreements, pledge agreements, mortgages, deeds of trust, trust deeds,
assignments, estoppel certificates, financing

                                      -10-
<PAGE>

statements and continuation thereof, termination statements, notices of
assignment, transfers, certificates, assurances and other instruments as the
requesting party reasonably may request from time to time in order to effectuate
the purposes of this Agreement, including, without limitation, to perfect and
maintain the validity, effectiveness and priority of any of the Instruments and
the liens and security interests intended to be created thereby and better to
assure, convey, grant, assign, transfer, preserve, protect and confirm unto PGGM
the rights granted now or hereafter intended to be granted under the
Instruments.

SECTION 12.       DESCRIPTIVE HEADINGS

                  The descriptive headings herein are inserted for convenience
only and are not intended to be part of or to affect the meaning or
interpretation of this Agreement.

SECTION 13.       COUNTERPARTS

                  This Agreement may be executed in counterparts, each of which
when so executed and delivered shall be an original, but all of such
counterparts shall together constitute one and the same instrument.

SECTION 14.       ENTIRE AGREEMENT; ASSIGNMENT

                  This Agreement (i) constitutes the entire agreement and
supersedes all prior agreements and understandings, both written and oral, among
the parties hereto with respect to the subject matter hereof and (ii) shall not
be assigned by operation of law or otherwise; PROVIDED, HOWEVER, following the
Effective Time, this Agreement may be assigned by EOP in the same circumstances
as under the Registration Rights Agreement.

SECTION 15.       NOTICES.

                  All notices, requests, claims, demands and other
communications under this Agreement shall be in writing and shall be delivered
personally, sent by overnight courier (providing proof of delivery) to the
parties or sent by telecopy (providing confirmation of transmission) at the
following addresses or telecopy numbers (or at such other address or telecopy
number for a party as shall be specified by like notice):

                                      -11-
<PAGE>

                      (a)   if to EOP or EOP Partnership, to:

                                 Equity Office Properties Trust
                                 EOP Operating Limited Partnership
                                 Two N. Riverside Plaza
                                 Chicago, IL 60606
                                 Attention: President
                                            Chief Counsel
                                 Fax No.: (312) 559-5021

                            with a copy to:

                                 Hogan & Hartson L.L.P.
                                 555 Thirteenth Street, N.W.
                                 Washington, D.C. 20004-1109
                                 Attention: J. Warren Gorrell, Jr., Esq.
                                            George P. Barsness, Esq.
                                 Fax No.: (202) 637-5910

                      (b)   if to Cornerstone or Cornerstone Partnership, to:

                                    Tower 56
                                    125 East 56th Street, 6th Floor New
                                    York, NY 10022 Attention: President
                                    Fax No.: (212) 605-7100

                            with a copy to:

                                    King & Spalding
                                    191 Peachtree Street
                                    Atlanta, GA 30303-1763
                                    Attention: William B. Fryer, Esq.
                                    Fax No.: (404) 572-5100

                      (c)   if to PGGM, to:

                                    Stichting Pensioenfonds voor de Gezondheid,
                                       Geestelijke en Maatschappelijke Belangen
                                    Utrechtseweg 44
                                    3714 HD Zeist
                                    The Netherlands
                                    Attention: Anneke C. van de Puttelaar
                                    Fax No.: 011 3130 696 3388

                                      -12-
<PAGE>

                            with a copy to:

                                    Richards & O'Neil, LLP
                                    885 Third Avenue
                                    New York, NY 10022
                                    Attention: Ann F. Chamberlain, Esq.
                                    Fax No.: (212) 750-9022


All notices shall be deemed given only when actually received.


SECTION 16.       GOVERNING LAW; CONSENT TO JURISDICTION; WAIVER OF JURY TRIAL

                  (a) This Agreement shall be governed by and construed in
accordance with the laws of the State of Maryland without regard to the
principles of conflicts of laws thereof.

                  (b) PGGM hereby submits and consents to non-exclusive personal
jurisdiction in any action, suit or proceeding arising out of this Agreement or
the transactions contemplated hereby in a federal court located in the State of
Maryland or in a Maryland state court. Any process, summons, notice or document
delivered by mail to the address set forth in Section 15 hereof shall be
effective service of process for any action, suit or proceeding in any Maryland
state court or any federal court located in the State of Maryland with respect
to any matters to which PGGM has submitted to jurisdiction in this Section 16.
PGGM irrevocably and unconditionally waives any objection to the laying of venue
of any action, suit or proceeding arising out of this Agreement or the
transactions contemplated hereby in any Maryland state court or any federal
court located in the State of Maryland, and hereby irrevocably and
unconditionally waives and agrees not to plead or claim in any such court that
any such action, suit or proceeding brought in any such court has been brought
in an inconvenient forum. PGGM IRREVOCABLY WAIVES TRIAL BY JURY IN ANY ACTION,
SUIT OR PROCEEDING WITH RESPECT TO THIS AGREEMENT.

SECTION 17.       SPECIFIC PERFORMANCE

                  The parties hereto agree that if any of the provisions of this
Agreement were not performed in accordance with their specific terms or were
otherwise breached, irreparable damage would occur, no adequate remedy at law
would exist and damages would be difficult to determine, and that the parties
shall be entitled to specific performance of the terms hereof, in addition to
any other remedy at law or equity. Nothing contained herein shall release any
party from any liability

                                      -13-
<PAGE>

arising from any breach of any of its representations, warranties, covenants or
agreements in this Agreement.

SECTION 18.       CAPACITY OF PGGM AND ITS OFFICERS, DIRECTORS AND EMPLOYEES.

                  PGGM has executed this Agreement solely in its capacity as a
stockholder of Cornerstone. Without limiting the foregoing, nothing in this
Agreement shall limit or affect any actions taken by any officer, director or
employee of PGGM in his capacity as an officer, director, employee or manager of
Cornerstone in connection with the exercise of Cornerstone's rights under the
Merger Agreement.



                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK.]


                                      -14-
<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have duly executed and
delivered this Voting Agreement, or have caused this Voting Agreement to be duly
executed and delivered in their names and on their behalf, as of the date first
written above.


                                     EQUITY OFFICE PROPERTIES TRUST

                                     By: /s/ STANLEY M. STEVENS
                                        ----------------------------------------
                                        Name: Stanley M. Stevens
                                        Title: Executive Vice President and
                                               Chief Legal Counsel


                                     EOP OPERATING LIMITED PARTNERSHIP

                                     By:  Equity Office Properties Trust, its
                                          general partner

                                          By: /s/ STANLEY M. STEVENS
                                             -----------------------------------
                                             Name: Stanley M. Stevens
                                             Title: Executive Vice President and
                                                    Chief Legal Counsel


                                     WCP SERVICES, INC.



                                     By: /s/ JOHN S. MOODY
                                        ----------------------------------------
                                        Name: John S. Moody
                                        Title: President


                                     PENSIOENFONDS VOOR DE
                                     GEZONDHEID, GEESTELIJKE EN
                                     MAATSCHAPPELIJKE BELANGEN


                                     By: /s/ JAN H.W.R. VAN DER VLIST
                                        ----------------------------------------
                                        Name: Jan H.W.R. van der Vlist
                                        Title: Attorney-in-fact

                                      -15-
<PAGE>




                                                                      SCHEDULE 1

<TABLE>
<CAPTION>
                           # OF SHARES
     NAME OF                    OF
   RECORD AND              CORNERSTONE
   BENEFICIAL                 COMMON                                    TERMS OF
     OWNER                    STOCK               PLEDGEE                PLEDGE
<S>                        <C>                 <C>                    <C>

PGGM                        45,779,703         Not Applicable         Not Applicable
</TABLE>


                                      -16-