<PAGE>
                                                                    Exhibit 10.1

                             STOCK OPTION AGREEMENT

                  THIS STOCK OPTION AGREEMENT (this "Agreement") is entered into
as of February 11, 2000, by and among CORNERSTONE PROPERTIES INC., a Nevada
corporation ("Cornerstone"), EQUITY OFFICE PROPERTIES TRUST, a Maryland real
estate investment trust ("EOP"), DEUTSCHE BANK AG der Deutschen Bank, a stock
corporation organized under German law ("Deutsche Bank"), and DEUTSCHER HEROLD
LEBENSVERSICHERUNGS-AG, a stock corporation organized under German law and an
indirect subsidiary of Deutsche Bank ("Deutscher Herold").

                  In consideration of the mutual covenants and agreements
contained herein and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, and intending to be legally bound
hereby, the parties hereto hereby agree as follows:


                                    ARTICLE I
                                THE STOCK OPTION

                  1.1 GRANT OF STOCK OPTION. Each of Deutsche Bank and Deutscher
Herold hereby grant to each of Cornerstone and EOP (each, a "Purchaser") an
irrevocable option (the "Stock Option") to purchase, subject to the terms
hereof, the number of fully paid and non-assessable shares of 7.0% Cumulative
Convertible Preferred Stock, with no par value, of Cornerstone ("Cornerstone
Preferred Stock") set forth on SCHEDULE A attached hereto opposite its name,
subject to adjustment pursuant to Section 5 hereof (such shares of Cornerstone
Preferred Stock held by Deutsche Bank and Deutscher Herold are referred to
collectively as the "Option Shares"), at a purchase price of $18.00 in cash per
share of Cornerstone Preferred Stock or, if the Merger Agreement (as defined
herein) is amended to increase the price per share of a share of Cornerstone
common stock, no par value, in excess of $18.00 per share, then at a purchase
price in cash equal to such greater price per share of Cornerstone common stock,
no par value, as is so specified in the Merger Agreement, as so amended, in each
case together with the dividends accrued and unpaid to the date of the Option
Closing (the "Purchase Price"), and in the manner set forth in Sections 1.2, 1.3
and 1.4 hereof.

                  1.2  EXERCISE OF THE STOCK OPTION.

                           (a) The Stock Option may be exercised by Cornerstone
or EOP, in whole but not in part, at any time or from time to time prior to
termination of the Stock Option in accordance with this Section 1.2 and Section
9.8 hereof.

                           (b) If at any time Cornerstone desires to exercise
the Stock Option, Cornerstone shall (1) deliver to each of Deutsche Bank and
Deutscher Herold a written notice (a "Cornerstone Exercise Notice," with the
date of the

<PAGE>

Cornerstone Exercise Notice being hereinafter called the "Cornerstone Notice
Date") notifying each of Deutsche Bank and Deutscher Herold of its exercise of
the Stock Option and (2) on the Cornerstone Notice Date, deliver to EOP a copy
of the Cornerstone Exercise Notice. The closing of such exercise by Cornerstone
of the Stock Option (the "Cornerstone Option Closing") shall occur on the third
business day immediately following the Cornerstone Notice Date at a time and at
a place in New York, New York designated by Cornerstone in the Cornerstone
Exercise Notice. "Business Day" means a business day in Frankfurt, Bonn and New
York.

                           (c) Subject to Section 1.2(d) hereof, if at any time
EOP desires to exercise the Stock Option, EOP shall (1) deliver to each of
Deutsche Bank and Deutscher Herold a written notice (an "EOP Exercise Notice,"
with the date of the EOP Exercise Notice being hereinafter called the "EOP
Notice Date") notifying each of Deutsche Bank and Deutscher Herold of its
exercise of the Stock Option and (2) on the EOP Notice Date, deliver to
Cornerstone a copy of the EOP Exercise Notice; PROVIDED, HOWEVER, that EOP shall
not be entitled to exercise the Stock Option or to deliver an EOP Exercise
Notice if Cornerstone has delivered a Cornerstone Exercise Notice in accordance
with the terms of this Agreement and the date for the Cornerstone Option Closing
corresponding to such Cornerstone Option Closing, as set forth in Section 1.2(b)
hereof, has not passed. Subject to Section 1.2(d) hereof, the closing of such
exercise by EOP of the Stock Option (the "EOP Option Closing") shall occur on
the ninth business day immediately following the EOP Notice Date at a time and
at a place in New York, New York designated by EOP in the EOP Exercise Notice.

                           (d) Upon receipt of an EOP Exercise Notice,
Cornerstone shall have the right, exercisable at any time prior to 5:00 p.m.,
New York time, on the second business day after the EOP Notice Date, to (1)
deliver to each of Deutsche Bank and Deutscher Herold a Cornerstone Exercise
Notice notifying each of Deutsche Bank and Deutscher Herold of its exercise of
the Stock Option and (2) on the Cornerstone Notice Date, deliver to EOP a copy
of the Cornerstone Exercise Notice. If Cornerstone so exercises the Stock
Option, the Cornerstone Option Closing shall occur on the third business day
immediately following the Cornerstone Notice Date at a time and at a place in
New York, New York designated by Cornerstone in the Cornerstone Exercise Notice.
Upon Cornerstone's purchase of the Option Shares pursuant thereto, the EOP
Exercise Notice delivered pursuant to Section 1.2(d) shall be deemed of no force
or effect.

                           (e) Upon receipt of a Cornerstone Exercise Notice,
each of Deutsche Bank and Deutscher Herold shall be obligated, provided that the
conditions specified in Section 1.3 hereto shall be satisfied, to deliver to
Cornerstone a certificate or certificates evidencing the Option Shares held by
it, in accordance with the terms of this Agreement, on the third business day
immediately following the Cornerstone Notice Date.

                                      -2-
<PAGE>

                           (f) On the date set for closing of any purchase of
Option Shares hereunder, each of Deutsche Bank and Deutscher Herold shall,
subject to satisfaction of the conditions specified in Section 1.3 hereof and
receipt of the Purchase Price therefor, deliver to Purchaser (i) a certificate
or certificates representing the Option Shares held by it, properly endorsed or
otherwise in proper form for transfer, and (ii) an irrevocable proxy in form
reasonably acceptable to Purchaser authorizing Purchaser or its designee to vote
such Option Shares on all matters submitted to a vote of the holders of
Cornerstone Preferred Stock.

                  1.3 CONDITIONS TO DELIVERY OF THE OPTION SHARES. The right of
each of Cornerstone or EOP to exercise the Stock Option and the obligation of
each of Deutsche Bank and Deutscher Herold to deliver the Option Shares held by
it upon the exercise of the Stock Option are subject to the following
conditions:

                           (a) Such delivery would not violate in any material
respect, or otherwise cause a material violation of, any material Law (as
defined herein), including, without limitation, under the Hart-Scott-Rodino
Antitrust Improvements Act of 1976, as amended ("Hart-Scott-Rodino"), applicable
to such exercise of Stock Option and the delivery of the Option Shares; and

                           (b) There shall be no preliminary or permanent
injunction or other order by any court of competent jurisdiction preventing or
prohibiting such exercise of the Stock Option or the delivery of Option Shares
in respect of such exercise.

                  1.4 CLOSING. At the Option Closing, Cornerstone or EOP, as
applicable, shall pay to each of Deutsche Bank and Deutscher Herold, upon
receipt of the certificate or certificates and proxy pursuant to Section 1.2(f),
the aggregate Purchase Price for Option Shares to be purchased by Cornerstone or
EOP, as applicable (net of any taxes required to be held pursuant to Section
1445 of the Internal Revenue Code of 1986, as amended), by wire transfer of
immediately available funds to an account(s) designated by each of Deutsche Bank
and Deutscher Herold at least two business days prior to the Option Closing.

                  1.5 ADJUSTMENTS UPON SHARE ISSUANCES, CHANGES IN
CAPITALIZATION, ETC. In the event of any change in the Cornerstone Preferred
Stock or in the number of outstanding shares of Cornerstone Preferred Stock by
reason of a stock dividend, split-up, recapitalization, combination, exchange of
shares or similar transaction or any other change in the corporate or capital
structure of Cornerstone (including, without limitation, the declaration or
payment of an extraordinary dividend in the form of cash, securities or other
property), the type and number of shares or securities to be delivered by
Deutsche Bank and Deutscher Herold upon exercise of the Stock Option shall be
adjusted appropriately.

                                      -3-
<PAGE>

                                   ARTICLE II
               REPRESENTATIONS AND WARRANTIES OF DEUTSCHE BANK AND
                                DEUTSCHER HEROLD

                  Each of Deutsche Bank and Deutscher Herold hereby represents
and warrants to each of Cornerstone and EOP as follows:

                  2.1 AUTHORITY RELATIVE TO THIS AGREEMENT. It has all requisite
corporate power and authority to execute and deliver this Agreement, to perform
its obligations hereunder and to consummate the transactions contemplated hereby
to be consummated by it. The execution and delivery of this Agreement by it and
the consummation by it of the transactions contemplated hereby have been duly
authorized by all necessary corporate action, and no other corporate proceedings
on the part of it are necessary to authorize the execution and delivery of this
Agreement or to consummate the transactions contemplated hereby. This Agreement
has been duly executed and delivered by it and constitutes the legal, valid and
binding obligations of it, enforceable against it in accordance with its terms,
subject to applicable bankruptcy, insolvency, moratorium or other similar Laws
relating to creditors' rights and general principles of equity.

                  2.2 NO LIENS. The Option Shares shall be delivered free and
clear of all security interests, liens, claims, pledges, options, rights of
first refusal, agreements, limitations on voting rights, charges and other
encumbrances of any nature whatsoever (other than this Agreement).

                  2.3 NO VIOLATION. The execution and delivery of this Agreement
by it do not, the performance of its obligations hereunder by it will not, the
consummation by it of the transactions contemplated to be performed by it
hereunder will not and the exercise by Cornerstone or EOP of the Stock Option
hereunder will not, (i) violate or conflict with any provision of any judgment,
order, decree, statute, law, ordinance, rule or regulation (collectively,
"Laws") in effect on the date of this Agreement and applicable to it (it being
understood that neither Deutsche Bank nor Deutscher Herold makes any
representation or warranty with respect to Hart-Scott-Rodino); (ii) require it
to obtain any consent, waiver, approval, license or authorization or permit of,
or make any filing with, or notification to, any governmental entities, based on
laws, rules, regulations and other requirements of governmental entities in
effect as of the date of this Agreement (it being understood that neither
Deutsche Bank nor Deutscher Herold makes any representation or warranty with
respect to Hart-Scott-Rodino); (iii) require the consent, waiver, approval,
license or authorization of any person (other than a governmental entity and
other than the property trustee (Deckungsstocktreuhander) of Deutscher Herold);
(iv) violate, conflict with or result in a breach of any obligation under, or
constitute a default (or an event which with notice or the lapse of time or both
would become a default) under, or give to others any rights of, or result in
any, termination, amendment, acceleration or cancellation of, or loss of any
benefit or

                                      -4-
<PAGE>

creation of a right of first refusal, or require any payment under, any
provision of any indenture, mortgage, note, bond, lien, lease, license,
agreement, contract, order, judgment, ordinance, permit or other instrument or
obligation to which it is a party or by which it is bound or subject to; or (v)
conflict with or violate the articles or certificate of incorporation or bylaws,
or the equivalent organizational documents, in each case as amended or restated,
of it.


                                   ARTICLE III
                  REPRESENTATIONS AND WARRANTIES OF CORNERSTONE

                  Cornerstone hereby represents and warrants to each of Deutsche
Bank and Deutscher Herold that it has all requisite corporate power and
authority to execute and deliver this Agreement, to perform its obligations
hereunder and to consummate the transactions contemplated hereby to be
consummated by Cornerstone. The execution and delivery of this Agreement by
Cornerstone have been duly authorized by the Board of Directors of Cornerstone.
This Agreement has been duly executed and delivered by Cornerstone and
constitutes the legal, valid and binding obligations of it, enforceable against
it in accordance with its terms, subject to applicable bankruptcy, insolvency,
moratorium or other similar laws relating to creditors' rights and general
principles of equity.


                                   ARTICLE IV
                            COVENANTS OF CORNERSTONE

                  Cornerstone hereby covenants and agrees that it shall acquire
Option Shares and/or other securities acquired upon its exercise of the Stock
Option for investment purposes only and not with a view to any distribution
thereof in violation of the Securities Act of 1933, as amended (the "Securities
Act"), and immediately shall cancel the Option Shares upon their purchase by
Cornerstone hereunder.


                                    ARTICLE V
                      REPRESENTATIONS AND WARRANTIES OF EOP

                  EOP hereby represents and warrants to each of Deutsche Bank
and Deutscher Herold that it has all requisite trust power and authority to
execute and deliver this Agreement, to perform its obligations hereunder and to
consummate the transactions contemplated hereby to be consummated by EOP. The
execution and delivery of this Agreement by EOP have been duly authorized by the
Board of Trustees of EOP. This Agreement has been duly executed and delivered by
EOP and constitutes the legal, valid and binding obligations of it, enforceable
against it in accordance with its terms, subject to applicable bankruptcy,
insolvency,

                                      -5-
<PAGE>

moratorium or other similar laws relating to creditors' rights and general
principles of equity.


                                   ARTICLE VI
                                COVENANTS OF EOP

                  EOP hereby covenants and agrees that it shall acquire Option
Shares and/or other securities acquired upon its exercise of the Stock Option
for investment purposes only and not with a view to any distribution thereof in
violation of the Securities Act, and shall not sell any such securities
purchased pursuant to this Agreement except in compliance with the Securities
Act.


                                   ARTICLE VII
                 COVENANTS OF DEUTSCHE BANK AND DEUTSCHER HEROLD

                  Each of Deutsche Bank and Deutscher Herold shall not, directly
or indirectly, and shall cause each of its affiliates not to, 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, Option Shares, (b) grant any proxies for any Option Shares, (c)
deposit any Option Shares into a voting trust or enter into a voting agreement
with respect to any Option Shares, or tender any Option Shares in a transaction,
or (d) take any action which is intended to have the effect of preventing or
disabling Deutsche Bank or Deutscher Herold from performing its obligations
under this Agreement; PROVIDED, HOWEVER, that nothing herein shall prevent the
sale, transfer, pledge or assignment of any of such Option Shares, provided that
the purchaser, transferee, pledgee or assignee thereof agrees in writing to be
bound by the terms of this Agreement. To enable either of the Purchasers to
enforce the transfer restrictions contained in this Article VII, each of
Deutsche Bank and Deutscher Herold hereby consents to the placing of
stop-transfer orders with the transfer agent of the Cornerstone Preferred Stock.

                                      -6-
<PAGE>

                                  ARTICLE VIII
                          WAIVER OF DISSENTERS' RIGHTS

                  Each of Deutsche Bank and Deutscher Herold hereby irrevocably
and forever waives any and all rights it may have under Chapter 92A of the
General Corporation Law of Nevada with respect to any of the transactions
contemplated by that certain Merger Agreement, dated as of February 11, 2000, by
and among EOP, EOP Partnership, Cornerstone and Cornerstone Partnership (the
"Merger Agreement"). Notwithstanding the foregoing, the foregoing waiver shall
be of no force or effect after December 31, 2000.


                                   ARTICLE IX
                                  MISCELLANEOUS

                  9.1 EXPENSES. Except as expressly provided herein to the
contrary, each party hereto shall pay all of its expenses in connection with the
transactions contemplated by this Agreement, including, without limitation, the
fees and expenses of its counsel and other advisors.

                  9.2 NOTICES. All notices and other communications given or
made pursuant hereto shall be in writing and shall be given (and shall be deemed
to have been duly given upon receipt) by delivery in person, by telecopy or
facsimile, by registered or certified mail (postage prepaid, return receipt
requested), or by a nationally recognized courier service to the parties at the
following addresses (or at such other address for a party as shall be specified
by like changes of address) or, if sent by telecopy or facsimile, to the parties
at the telecopier numbers specified below:

<TABLE>
<S>                                          <C>
                  If to Cornerstone:         Cornerstone Properties Inc.
                                             126 East 56th Street
                                             New York, NY  10022
                                             Attention: Lee Van Boven, Esq.
                                             Telecopier: 650-345-8264

                  With a copy                King & Spalding
                  (which shall not           191 Peachtree Street
                  constitute notice) to:     Atlanta, GA 30303-1763
                                             Attention: William B. Fryer, Esq.
                                             Telecopier: 404-572-5100
</TABLE>

                                      -7-
<PAGE>

<TABLE>
<S>                                          <C>
                  If to EOP:                 Equity Office Properties Trust
                                             Two North Riverside Plaza
                                             Chicago, IL 60606
                                             Attention: Stanley M. Stevens, Esq.
                                             Telecopier: 312-559-5021

                  With copies                Hogan & Hartson L.L.P.
                  (which shall not           555 13th Street, N.W.
                  constitute notice) to:     Washington, D.C. 20004
                                             Attention: J. Warren Gorrell, Jr., Esq.
                                                        George P. Barsness, Esq.
                                             Telecopier: 202-637-5910


                  If to Deutsche Bank:       Deutsche Bank AG
                                             Group Investments
                                             Taunusanlage 12
                                             60325 Frankfurt am Main
                                             GERMANY
                                             Attention: Lutz Robra
                                             Telecopier: 49 69 910-43583

                  With a copy                White & Case LLP
                  (which shall not           1155 Avenue of the Americas
                  constitute notice) to:     New York, NY 10036
                                             Attention: Duane D. Wall, Esq.
                                             Telecopier: (212) 819-8200


                  If to Deutscher Herold:    Deutscher Herold Lebensversicherungs-AG
                                                 der Deutschen Bank
                                             Poppelsdorfer Allee 25-33
                                             53115 Bonn
                                             GERMANY
                                             Attention: Mr. J. Klahn
                                             Telecopier: 49 228 268 3838

                  With copies                 White & Case LLP
                  (which shall not            1155 Avenue of the Americas
                  constitute notice) to:      New York, NY 10036
                                              Attention: Duane D. Wall, Esq.
                                              Telecopier: (212) 819-8200
</TABLE>

                                      -8-
<PAGE>

                  9.3 ENTIRE AGREEMENT. This Agreement constitutes the entire
agreement between the parties and supersedes all prior agreements and
understandings, both written and oral, between the parties, or any of them, with
respect to the subject matter hereof.

                  9.4 ASSIGNMENT. Neither this Agreement nor any rights or
obligations hereunder shall be assigned by operation of law or otherwise,
without the prior written consent of the other party hereto; PROVIDED, HOWEVER,
that EOP may assign this Agreement or any of its rights or obligations hereunder
to EOP Operating Limited Partnership, a Delaware limited partnership ("EOP
Partnership"), without the consent of any other party hereto; PROVIDED, FURTHER,
HOWEVER, that Cornerstone may assign this Agreement or any of its rights or
obligations hereunder to Cornerstone Properties Limited Partnership, a Delaware
limited partnership ("Cornerstone Partnership"), without the consent of any
other party hereto (PROVIDED, HOWEVER, that Cornerstone Partnership agrees to
cause Cornerstone to cancel the Option Shares immediately upon any purchase by
Cornerstone Partnership hereunder).

                  9.5 GOVERNING LAW. This Agreement, and all matters relating
hereto, shall be governed by, and construed in accordance with the laws of the
State of Maryland, regardless of the laws that might govern under applicable
principles of conflict of law theory.

                  9.6 CONSENT TO JURISDICTION; WAIVER OF JURY TRIAL. Each of
Deutsche Bank and Deutscher Herold 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 (and for no other purposes) 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 9.2 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 Deutsche Bank or
Deutscher Herold has submitted to jurisdiction in this Section 9.6. Each of
Deutsche Bank and Deutscher Herold 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. EACH OF DEUTSCHE BANK AND DEUTSCHER
HEROLD IRREVOCABLY WAIVES TRIAL BY JURY IN ANY ACTION, SUIT OR PROCEEDING WITH
RESPECT TO THIS AGREEMENT.

                  9.7 INJUNCTIVE RELIEF. The parties agree that in the event of
a breach of any provision of this Agreement irreparable damage would occur, the
aggrieved

                                      -9-
<PAGE>

party would be without an adequate remedy at law and damages would be difficult
to determine. The parties therefore agree that in the event of a breach of any
provision of this Agreement, the aggrieved party may elect to institute and
prosecute proceedings in any court of competent jurisdiction to enforce specific
performance or to enjoin the continuing breach of such provision. By seeking or
obtaining such relief, the aggrieved party shall not be precluded from seeking
or obtaining any other relief to which it may be entitled at law or in equity.

                  9.8 TERMINATION. The Stock Option shall terminate upon the
earlier of: (i) the consummation of the Cornerstone Option Closing or the EOP
Option Closing hereunder; (ii) the date which is the tenth day after the
termination of the Merger Agreement; or (iii) December 31, 2000.

                  9.9 COUNTERPARTS. This Agreement may be executed in any number
of counterparts, each of which shall be deemed to be an original and all of
which together shall constitute one and the same document.

                  9.10 SEVERABILITY. Any term or provision of this Agreement
which is invalid or unenforceable in any jurisdiction shall, as to such
jurisdiction, be ineffective to the extent of such invalidity or
unenforceability without rendering invalid or unenforceable the remaining terms
and provisions of this Agreement or affecting the validity or enforceability of
any of the terms or provisions of this Agreement in any other jurisdiction. If
any provision of this Agreement is so broad as to be unenforceable, such
provision shall be interpreted to be only so broad as to make it enforceable.

                  9.11 FURTHER ASSURANCES. Each party hereto shall execute and
deliver all such further documents and take all such further action as may be
necessary in order to consummate the transactions contemplated by this
Agreement.

                  9.12 THIRD PARTY BENEFICIARIES. Nothing in this Agreement,
expressed or implied, shall be construed to give any person other than the
parties hereto and their successors and permitted assigns any legal or equitable
right, remedy or claim under or by reason of this Agreement or any provision
contained herein.

                  9.13 AMENDMENT AND MODIFICATION. This Agreement may be
amended, modified and supplemented only by a written document executed by
Cornerstone, EOP, Deutsche Bank and Deutscher Herold.

                  9.14 TREATMENT OF CORNERSTONE PREFERRED STOCK IN THE MERGER.
Each share of Cornerstone Preferred Stock shall be converted in the Merger into
the right to receive $18.00, together with accrued and unpaid dividends to the
Effective Time of the Merger, in cash, as described in Section 1.10(b)(iii) of
the Merger Agreement.

                  9.15 PUT RIGHT. In the event that the Stock Option is not
exercised within two business days after the Effective Time of the Merger, each
of Deutsche

                                      -10-
<PAGE>

Bank and Deutscher Herold shall have the right to require EOP to purchase the
Option Shares in the same manner and with the same effect as if EOP shall have
given an EOP Exercise Notice, whereupon EOP shall be required to purchase such
Option Shares in accordance with this Agreement.

                                      -11-

<PAGE>

                  IN WITNESS WHEREOF, the parties have caused this Agreement to
be executed as of the date first written above by their respective officers
thereunto duly authorized.


                                             CORNERSTONE PROPERTIES INC.

                                             By:
                                                --------------------------------
                                                Name:
                                                Title:



                                             EQUITY OFFICE PROPERTIES TRUST

                                             By:
                                                --------------------------------
                                                Name:
                                                Title:



                                             DEUTSCHE BANK AG
                                             By:
                                                --------------------------------
                                                Name:
                                                Title:



                                             DEUTSCHER HEROLD
                                             LEBENSVERSICHERUNGS-AG
                                             By:
                                                --------------------------------
                                                Name:
                                                Title:

                                      -12-
<PAGE>

                                   SCHEDULE A

                           Cornerstone Properties Inc.
                    7% Cumulative Convertible Preferred Stock


Deutsche Bank AG                                         2,249,053 shares

Deutscher Herold Lebensversicherungs-AG                    781,250 shares