Document:

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                                                                   EXHIBIT 10.26

                       MEMBERSHIP INTEREST SALE AGREEMENT
                                    (KING I)

                                 BY AND BETWEEN

                          AETNA LIFE INSURANCE COMPANY,
                            A CONNECTICUT CORPORATION

                                    AS SELLER

                                       AND

                               COLUMBIA EQUITY LP,
                         A VIRGINIA LIMITED PARTNERSHIP,

                                  AS PURCHASER

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<S>                                                                                                             <C>
ARTICLE I  THE SALE...............................................................................................1

         1.1        Sale of Membership Interest...................................................................1
         1.2        Purchase Price................................................................................1

ARTICLE II  REPRESENTATIONS AND COVENANTS.........................................................................2

         2.1        Representations by Purchaser..................................................................2
         2.2        Representations by Seller.....................................................................3
         2.3        Covenants of Purchaser........................................................................4
         2.4        Covenants of Seller...........................................................................4

ARTICLE III  Conditions Precedent to the Closing..................................................................5

         3.1        Conditions to Purchaser's Obligations.........................................................5
         3.2        Conditions to Seller's Obligations............................................................6

ARTICLE IV  Closing and Closing Documents.........................................................................7

         4.1        Closing.......................................................................................7
         4.2        Seller's Deliveries...........................................................................7
         4.3        Purchaser's Deliveries........................................................................8
         4.4        Fees and Expenses; Closing Costs..............................................................8
         4.5        Distributions Prior to Closing................................................................8

ARTICLE V  Miscellaneous..........................................................................................9

         5.1        Notices.......................................................................................9
         5.2        Entire Agreement; Modifications and Waivers; Cumulative Remedies.............................10
         5.3        Exhibits.....................................................................................10
         5.4        Successors and Assigns.......................................................................10
         5.5        Article Headings.............................................................................10
         5.6        Governing Law................................................................................11
         5.7        Counterparts.................................................................................11
         5.8        Survival.....................................................................................11
         5.9        Severability.................................................................................11
         5.10       Attorneys' Fees..............................................................................11

EXHIBITS
         A        Assignment and Assumption Agreement
         B        First Amendment to Amended and Restated Operating Agreement of King I, LLC
</TABLE>

<PAGE>

                       MEMBERSHIP INTEREST SALE AGREEMENT

        THIS MEMBERSHIP INTEREST SALE AGREEMENT (this "Agreement") is made as of
this 25th day of May, 2005 by and between Aetna Life Insurance Company, a
Connecticut corporation ("Seller"); and Columbia Equity, LP, a Virginia limited
partnership ("Purchaser").

                                    RECITALS

        A.      King I LLC, a Virginia limited liability company (the "LLC") is
the owner of certain land located at 1800 Diagonal Road, Alexandria, Virginia
(the "Land") and the office building and related improvements located thereon
(the "Improvements"), which Land and Improvements (collectively, the "Property")
are more commonly known as King Street Station I.

        B.      Seller is the record and beneficial owner of eighty-five percent
(85%) ("Seller's Share") of the membership interests in the LLC.

        C.      Seller desires to sell a thirty-five percent membership interest
in the LLC (the "Membership Interest") to Purchaser, on the terms and conditions
hereinafter set forth.

        D.      Purchaser desires to purchase the Membership Interest from
Seller, on the terms and conditions hereinafter set forth.

                                    AGREEMENT

        NOW, THEREFORE, for and in consideration of the mutual covenants herein
contained, the parties hereto agree as follows:

                                    ARTICLE I
                                    THE SALE

        1.1     Sale of Membership Interest. Seller agrees to sell, transfer,
assign and convey the Membership Interest to Purchaser, and Purchaser agrees to
purchase and accept transfer of the Membership Interest pursuant to the terms
and conditions set forth in this Agreement. The Membership Interest shall be
transferred to Purchaser free and clear of all liens, encumbrances, security
interests, prior assignments or conveyances, conditions, restrictions, voting
agreements, claims, and any other matters affecting title thereto (other than
the LLC's operating agreement (the "LLC Operating Agreement")).

        1.2     Purchase Price. The purchase price (the "Purchase Price") for
which Seller agrees to sell and assign the Membership Interest to Purchaser, and
which Purchaser agrees to pay to Seller, subject to the terms of this Agreement,
shall be Five Million Two Hundred Fifty Thousand Dollars ($5,250,000).

                                       -1-
<PAGE>

                                   ARTICLE II
                          REPRESENTATIONS AND COVENANTS

        2.1     Representations by Purchaser. Purchaser hereby represents and
warrants to Seller that the following statements are true, correct, and complete
in every material respect as of the date of this Agreement and will be true,
correct, and complete as of the Closing Date:

                (a)     Organization and Power. Purchaser is duly organized and
validly existing as a limited partnership under the laws of the Commonwealth of
Virginia, and has full right, power, and authority to enter into this Agreement
and to perform all of its obligations under this Agreement; and, the execution
and delivery of this Agreement and the performance by Purchaser of its
obligations under this Agreement have been duly authorized by all requisite
action of Purchaser and require no further action or approval of Purchaser's
partners or of any other individuals or entities in order to constitute this
Agreement as a binding and enforceable obligation of Purchaser.

                (b)     Noncontravention. Neither the entry into nor the
performance of, or compliance with, this Agreement by Purchaser has resulted, or
will result, in any violation of, or default under, or result in the
acceleration of, any obligation under the partnership agreement of Purchaser, or
any mortgage, indenture, lien agreement, note, contract, permit, judgment,
decree, order, restrictive covenant, statute, rule, or regulation applicable to
Purchaser.

                (c)     Litigation. To Seller's actual knowledge, there is no
action, suit, or proceeding, pending or known to be threatened, against or
affecting Purchaser in any court or before any arbitrator or before any federal,
state, municipal, or other governmental department, commission, board, bureau,
agency or instrumentality which (i) in any manner raises any question affecting
the validity or enforceability of this Agreement, (ii) would reasonably be
expected to materially and adversely affect the business, financial position, or
results of operations of Purchaser, (iii) would reasonably be expected to
materially and adversely affect the ability of Purchaser to perform its
obligations hereunder, or under any document to be delivered pursuant hereto.

                (d)     Consents. Each consent, approval, authorization, order,
license, certificate, permit, registration, designation, or filing by or with
any governmental agency or body necessary for the execution, delivery, and
performance of this Agreement or the transactions contemplated hereby by
Purchaser has been obtained.

                (e)     Bankruptcy with respect to Purchaser. No Act of
Bankruptcy has occurred with respect to Purchaser. As used herein, "Act of
Bankruptcy" shall mean if a party hereto shall (A) apply for or consent to the
appointment of, or the taking of possession by, a receiver, custodian, trustee
or liquidator of itself or of all or a substantial part of its property, (B)
admit in writing its inability to pay its debts as they become due, (C) make a
general assignment for the benefit of its creditors, (D) file a voluntary
petition or commence a voluntary case or proceeding under the Federal Bankruptcy
Code (as now or hereafter in effect), (E) be adjudicated bankrupt

                                      -2-
<PAGE>

or insolvent, (F) file a petition seeking to take advantage of any other law
relating to bankruptcy, insolvency, reorganization, winding-up or composition or
adjustment of debts, (G) fail to controvert in a timely and appropriate manner,
or acquiesce in writing to, any petition filed against it in an involuntary case
or proceeding under the Federal Bankruptcy Code (as now or hereafter in effect),
or (H) take any action for the purpose of effecting any of the foregoing.

                (f)     Brokerage Commission. Purchaser has not engaged the
services of, nor has it or will it or Seller become liable to, any real estate
agent, broker, finder or any other person or entity for any brokerage or
finder's fee, commission or other amount with respect to the transactions
described herein on account of any action by Purchaser. Purchaser hereby agrees
to indemnify and hold Seller and its employees, directors, members, partners,
affiliates and agents harmless against any claims, liabilities, damages or
expenses arising out of a breach of the foregoing. This indemnification shall
survive Closing or any termination of this Agreement.

        2.2     Representations by Seller. Seller hereby represents and warrants
unto Purchaser that each and every one of the following statements is true,
correct, and complete in every material respect as of the date of this Agreement
and will be true, correct, and complete as of the Closing Date:

                (a)     Organization and Power. Seller is duly organized,
validly existing, and in good standing as a corporation under the laws of the
State of Connecticut. Seller has full right, power, and authority to enter into
this Agreement and to perform all of its obligations under this Agreement; and
the execution and delivery of this Agreement and the performance by Seller of
its obligations hereunder have been duly authorized by all requisite action of
Seller and require no further action or approval of Seller's members or managers
or of any other individuals or entities in order to constitute this Agreement as
a binding and enforceable obligation of Seller.

                (b)     Noncontravention. Neither the entry into nor the
performance of, or compliance with, this Agreement by Seller has resulted, or
will result, in any violation of, or default under, or result in the
acceleration of, any obligation under any limited liability company agreement,
operating agreement, regulation, mortgage, indenture, lien agreement, note,
contract, permit, judgment, decree, order, restrictive covenant, statute, rule,
or regulation applicable to Seller or to the Membership Interest.

                (c)     Litigation. There is no action, suit, claim, or
proceeding pending against or threatened against or affecting Seller or the
Membership Interest in any court, or before any arbitrator, or before any
federal, state, municipal or other governmental department, commission, board,
bureau, agency or instrumentality which (A) in any manner raises any question
affecting the validity or enforceability of this Agreement, (B) would reasonably
be expected to materially and adversely affect the business, financial position
or results of operations of Seller, (C) would reasonably be expected to
materially and adversely affect the ability of Seller to perform its obligations
hereunder, or under any document to be delivered pursuant hereto, (D) would
reasonably be expected to create a lien on the Membership Interest, any part
thereof, or any

                                      -3-
<PAGE>

interest therein, or (E) would reasonably be expected to adversely affect the
Membership Interest, any part thereof, or any interest therein.

                (d)     Good Title. (A) Seller has good title on the date hereof
and will have good title on the Closing Date to the Membership Interest (other
than the LLC Operating Agreement), (B) the Membership Interest on the date
hereof is and on the Closing Date will be free and clear of all liens,
encumbrances, pledges, voting agreements and security interests whatsoever
(other than the LLC Operating Agreement), and (C) Seller has not granted any
other person or entity an option to purchase or a right of first refusal upon
the Membership Interest nor are there any agreements or understandings between
Seller and any other person or entity with respect to the disposition of the
Membership Interest (other than the LLC Operating Agreement).

                (e)     No Consents. Each consent, approval, authorization,
order, license, certificate, permit, registration, designation, or filing by or
with, any governmental agency or body necessary of the execution, delivery, and
performance of this Agreement or the transactions contemplated hereby by Seller
has been obtained or will be obtained on or before the Closing Date.

                (f)     Bankruptcy with respect to Seller. No Act of Bankruptcy
has occurred with respect to Seller.

                (g)     Brokerage Commission. Seller has not engaged the
services of, nor has it or will it or Purchaser become liable to, any real
estate agent, broker, finder or any other person or entity for any brokerage or
finder's fee, commission or other amount with respect to the transactions
described herein on account of any action by Seller. Seller hereby agrees to
indemnify and hold Purchaser and its employees, directors, members, partners,
affiliates and agents harmless against any claims, liabilities, damages or
expenses arising out of a breach of the foregoing. This indemnification shall
survive Closing or any termination of this Agreement.

                (h)     Default. To Seller's actual knowledge, Seller is not in
default under the LLC Operating Agreement.

        2.3     Covenants of Purchaser. Purchaser agrees as follows:

                (a)     Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Purchaser shall perform, execute, and deliver
or cause to be performed, executed, and delivered at the Closing or after the
Closing, any and all further acts, instruments, and agreements and provide such
further assurances as Seller may reasonably require to consummate the
transactions contemplated hereunder.

        2.4     Covenants of Seller. Seller agrees as follows:

                (a)     Actions Regarding Membership Interest. Except as
otherwise permitted hereby, from the date hereof until the Closing Date, Seller
shall use reasonable commercial

                                      -4-
<PAGE>

efforts not to take any action or fail to take any action reasonably within
Seller's control the result of which would (1) have a material adverse effect on
the Membership Interest, the Property, Seller's ability to sell, transfer,
assign and convey the Membership Interest to Purchaser or Purchaser's ability to
continue the ownership thereof after the Closing Date or (2) cause any of the
representations and warranties contained in Section 2.2 to be untrue as of the
Closing Date.

                (b)     Confidentiality. Seller acknowledges that matters
relating to the Columbia Equity Trust, Inc. (the "REIT"), the initial public
offering of REIT securities (the "IPO") and this Agreement (collectively, the
"Information") are confidential in nature. Therefore, Seller covenants and
agrees to keep the Information confidential and will not (except as required by
applicable law, regulation or legal process, and only after compliance with the
provisions of this Section 2.4) prior to the IPO, without Purchaser's prior
written consent, disclose any Information in any manner whatsoever; provided,
however, that the Information may be revealed only to Seller's employees, legal
counsel and financial advisors, each of whom shall be informed of the
confidential nature of the Information. In the event that Seller or its key
employees, legal counsel or financial advisors (collectively, the "Information
Group") are requested pursuant to, or required by, applicable law, regulation or
legal process to disclose any of the Information, the applicable member of the
Information Group will notify Purchaser promptly so that it may seek a
protective order or other appropriate remedy or, in its sole discretion, waive
compliance with the terms of this Section 2.4. Seller acknowledges that remedies
at law may be inadequate to protect Purchaser or the REIT against any actual or
threatened breach of this Section 2.4, and, without prejudice to any other
rights and remedies otherwise available, Seller agrees to the granting of
injunctive relief in favor of the REIT and/or Purchaser without proof of actual
damages.

                (c)     Other Members' Transactions. Seller hereby consents to
the assignment by each of the other members of the LLC of such other members'
membership interest in the LLC to Purchaser and agrees to the admission of
Purchaser as a member in the LLC.

                (d)     Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Seller shall perform, execute, and deliver or
cause to be performed, executed, and delivered at the Closing or after the
Closing, any and all further acts, instruments, and agreements and provide such
further assurances as Purchaser may reasonably require to consummate the
transactions contemplated hereunder.

                                  ARTICLE III
                       CONDITIONS PRECEDENT TO THE CLOSING

        3.1     Conditions to Purchaser's Obligations. In addition to any other
conditions set forth in this Agreement, Purchaser's obligation to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.1, all of which shall be
conditions precedent to Purchaser's obligations under this Agreement.

                                      -5-
<PAGE>

                (a)     Seller's Obligations. Seller shall have performed all
obligations of Seller hereunder which are to be performed prior to Closing, and
shall have delivered or caused to be delivered to Purchaser, all of the
documents and other information required of Seller pursuant to Section 4.2.

                (b)     Seller's Representations and Warranties. Seller's
representations and warranties set forth in Section 2.2 shall be true and
correct in all material respects as if made again on the Closing Date, and
Seller shall have executed and delivered to Purchaser at Closing a certificate
to the foregoing effect.

                (c)     No Injunction. On the Closing Date, there shall be no
effective injunction, writ, preliminary restraining order or other order issued
by a court of competent jurisdiction restraining or prohibiting the consummation
of the transactions contemplated hereby.

                (d)     Completion of IPO. The IPO shall have been completed.

                (e)     Other Members Transactions. All of the other members of
the LLC shall have sold their interests in the LLC to Purchaser or consented to
the Amended LLC Operating Agreement (as hereinafter defined) and to the
transactions contemplated by this Agreement.

                (f)     Mortgage Lender Consent. The holder of the mortgage loan
secured by the Property shall have consented to the transactions contemplated by
this Agreement.

                (g)     Purchaser's General Partner. The REIT shall be the sole
general partner of the Purchaser and have control over the management and
operations of the Purchaser.

                (h)     Closing. The Closing shall have occurred on or prior to
June 30, 2005.

        3.2     Conditions to Seller's Obligations. In addition to any other
conditions set forth in this Agreement, Seller's obligations to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.2, all of which shall be
conditions precedent to Seller's obligations under this Agreement.

                (a)     Purchaser's Obligations. Purchaser shall have performed
all obligations of Purchaser hereunder which are to be performed prior to
Closing, and shall have delivered or caused to be delivered to Seller, all of
the documents and other information required of Purchaser pursuant to Section
4.3.

                (b)     Purchaser's Representations and Warranties. Purchaser's
representations and warranties set forth in Section 2.1 shall be true and
correct in all material respects as if made again on the Closing Date, and
Purchaser shall have executed and delivered to Seller at Closing a certificate
to the foregoing effect.

                                      -6-
<PAGE>

                (c)     No Injunction. On the Closing Date, there shall be no
effective injunction, writ, preliminary restraining order or other order issued
by a court of competent jurisdiction restraining or prohibiting the consummation
of the transactions contemplated hereby.

                (d)     Completion of IPO. The IPO shall have been completed.

                (e)     Other Members Transactions. All of the other members of
the LLC shall have sold their interests in the LLC to Purchaser or consented to
the Amended LLC Operating Agreement (as hereinafter defined) and to the
transactions contemplated by this Agreement.

                (f)     Mortgage Lender Consent. The holder of the mortgage loan
secured by the Property shall have consented to the transactions contemplated by
this Agreement.

                (g)     Purchaser's General Partner. The REIT shall be the sole
general partner of the Purchaser and have control over the management and
operations of the Purchaser.

                (h)     Closing. The Closing shall have occurred on or prior to
June 30, 2005.

                                   ARTICLE IV
                         CLOSING AND CLOSING DOCUMENTS

        4.1     Closing. The consummation and closing (the "Closing") of the
transactions contemplated under this Agreement shall take place at the offices
of Hunton & Williams LLP, Washington, D.C., or such other place as is mutually
agreeable to the parties, on the date of the closing of the IPO (the "Closing
Date"), or as otherwise set by agreement of the parties; provided, however, that
this Agreement shall terminate if Closing does not occur prior to June 30, 2005.

        4.2     Seller's Deliveries. At the Closing, Seller shall deliver the
following to Purchaser in addition to all other items required to be delivered
to Purchaser by Seller:

                (a)     Assignment of Membership Interest. The Assignment and
Assumption Agreement, in substantially the form of Exhibit A attached hereto
(the "Assignment and Assumption Agreement") executed by Seller.

                (b)     Amended LLC Operating Agreement. The Amended and
Restated Operating Agreement of the LLC, in substantially the form of Exhibit B
attached hereto (the "Amended LLC Operating Agreement") executed by Seller.

                (c)     Authority Documents. Evidence satisfactory to Purchaser
that the person or persons executing the closing documents on behalf of Seller
has full right, power, and authority to do so.

                (d)     FIRPTA Certificate. An affidavit from Seller certifying
pursuant to Section 1445 of the Internal Revenue Code that Seller is not a
foreign

                                      -7-
<PAGE>

corporation, foreign partnership, foreign trust, foreign estate or foreign
person (as those terms are defined in the Internal Revenue Code and the Income
Tax Regulations promulgated thereunder), in form and substance satisfactory to
Purchaser.

                (e)     Certificate of Representations and Warranties. The
certificate required by Section 3.1(b).

                (f)     Other Documents. Any other document or instrument
reasonably requested by Purchaser or required hereby.

        4.3     Purchaser's Deliveries. At the Closing, Purchaser shall deliver
the following to Seller:

                (a)     Purchase Price. The Purchase Price.

                (b)     Assumption of Membership Interest. The Assignment and
Assumption Agreement executed by Purchaser.

                (c)     Amended LLC Operating Agreement. The Amended LLC
Operating Agreement executed by Purchaser (and any other members thereof other
than Seller).

                (d)     Authority Documents. Evidence satisfactory to Seller
that the person or persons executing the closing documents on behalf of
Purchaser have full right, power, and authority to do so.

                (e)     Certificate of Representations and Warranties. The
certificate required by Section 3.2(b).

                (f)     Other Documents. Any other document or instrument
reasonably requested by Seller or required hereby.

        4.4     Fees and Expenses; Closing Costs. Purchaser shall pay all fees,
expenses and closing costs relating to the transactions contemplated by this
Agreement; provided however, that Seller shall pay its own attorneys' and
consultants' fees and expenses.

        4.5     Distributions Prior to Closing. Purchaser shall pay funds in the
amount of the Purchase Price to Seller without any setoffs or adjustments.
Purchaser and Seller agree that, after the date hereof and through the Closing
Date, the LLC will continue to be operated in the normal course of business
consistent with the LLC Operating Agreement and the Approved Budget (as defined
in the LLC Operating Agreement) for 2005. On or before Closing, cash held by or
for the benefit of the LLC (excluding lender-held reserves in excess of funds
due to the LLC for capital costs incurred prior to Closing) in excess of
$900,000 shall be distributed to the LLC's members in accordance with the
provisions of the LLC Operating Agreement.

                                      -8-
<PAGE>

                                   ARTICLE V
                                  MISCELLANEOUS

        5.1     Notices. Any notice provided for by this Agreement and any other
notice, demand, or communication required hereunder shall be in writing and
either delivered in person (including by confirmed facsimile transmission) or
sent by registered or certified mail or overnight courier, return receipt
requested, in a sealed envelope, postage prepaid, and addressed to the party for
which such notice, demand or communication is intended at such party's address
as set forth in this Section. All notices to Purchaser shall be addressed as
follows:

        PURCHASER:

                Columbia Equity LP
                c/o Carr Capital Corporation
                1750 H Street, NW, Suite 500
                Washington, DC 20005
                Telephone: (202) 303-3060
                Facsimile: (202) 303-3078
                Attn: Oliver T. Carr, III

        WITH COPIES TO:

                Hunton & Williams LLP
                1900 K Street, Suite 1200
                Washington, DC 20006
                Telephone: (202) 778-2221
                Facsimile: (202) 778-2201
                Attn: John M. Ratino

Seller's address for all purposes under this Agreement shall be the following:

        SELLER:

                Aetna Life Insurance Company
                151 Farmington Avenue, RT 11
                Hartford, Connecticut 06156-9642
                Telephone: (860) 273-1781
                Facsimile: (860) 273-1620
                Attn: Peter S. Atwood

                                      -9-
<PAGE>

        WITH COPIES TO:

                Bingham McCutchen LLP
                One State Street
                Hartford, Connecticut 06103
                Telephone: (860) 240-2759
                Facsimile: (860) 240-2800
                Attn: R. Jeffrey Smith, Esq.

                Any address or name specified above may be changed by a notice
                given by the addressee to the other party. Any notice, demand or
                other communication shall be deemed given and effective as of
                the date of delivery in person or receipt set forth on the
                return receipt. The inability to deliver because of changed
                address of which no notice was given, or rejection or other
                refusal to accept any notice, demand or other communication,
                shall be deemed to be receipt of the notice, demand or other
                communication as of the date of such attempt to deliver or
                rejection or refusal to accept.

        5.2     Entire Agreement; Modifications and Waivers; Cumulative
Remedies. This Agreement supersedes any existing letter of intent between the
parties, constitutes the entire agreement among the parties hereto and may not
be modified or amended except by instrument in writing signed by the parties
hereto, and no provisions or conditions may be waived other than by a writing
signed by the party waiving such provisions or conditions. No delay or omission
in the exercise of any right or remedy accruing to Seller or Purchaser upon any
breach under this Agreement shall impair such right or remedy or be construed as
a waiver of any such breach theretofore or thereafter occurring. The waiver by
Seller or Purchaser of any breach of any term, covenant, or condition herein
stated shall not be deemed to be a waiver of any other breach, or of a
subsequent breach of the same or any other term, covenant, or condition herein
contained. All rights, powers, options, or remedies afforded to Seller or
Purchaser either hereunder or by law shall be cumulative and not alternative,
and the exercise of one right, power, option, or remedy shall not bar other
rights, powers, options, or remedies allowed herein or by law, unless expressly
provided to the contrary herein.

        5.3     Exhibits. All exhibits referred to in this Agreement and
attached hereto are hereby incorporated in this Agreement by reference.

        5.4     Successors and Assigns. Except as set forth in this Article,
this Agreement may not be assigned by Purchaser or Seller without the prior
approval of the other party hereto. This Agreement shall be binding upon, and
inure to the benefit of, Seller, Purchaser, and their respective legal
representatives, successors, and permitted assigns.

        5.5     Article Headings. Article headings and article and section
numbers are inserted herein only as a matter of convenience and in no way
define, limit, or prescribe the scope or intent of this Agreement or any part
hereof and shall not be considered in interpreting or construing this Agreement.

                                      -10-
<PAGE>

        5.6     Governing Law. This Agreement shall be construed and interpreted
in accordance with the laws of the Commonwealth of Virginia, without regard to
conflicts of laws principles. 5.7 Counterparts. This Agreement may be executed
in any number of counterparts and by any party hereto on a separate counterpart,
each of which when so executed and delivered shall be deemed an original and all
of which taken together shall constitute but one and the same instrument.

        5.8     Survival. All representations and warranties contained in this
Agreement, and all covenants and agreements contained in the Agreement which
contemplate performance after the Closing Date shall survive the Closing.

        5.9     Severability. In case any one or more of the provisions
contained in this Agreement shall for any reason be held to be invalid, illegal,
or unenforceable in any respect, such invalidity, illegality, or
unenforceability shall not affect any other provision hereof, and this Agreement
shall be construed as if such invalid, illegal, or unenforceable provision had
never been contained herein.

        5.10    Attorneys' Fees. Should a party employ an attorney or attorneys
to enforce any of the provisions hereof or to protect its interest in any manner
arising under this Agreement, or to recover damages for breach of this
Agreement, any non-prevailing party in any action pursued in a court of
competent jurisdiction (the finality of which is not legally contested) shall
pay to the prevailing party all reasonable costs, damages, and expenses,
including reasonable attorneys' fees, expended or incurred in connection
therewith.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK.]
                     [SIGNATURES APPEAR ON FOLLOWING PAGES.]

                                      -11-
<PAGE>

        IN WITNESS WHEREOF, this Agreement has been entered into effective as of
the date first above written.

                                     SELLER:

                                     Aetna Life Insurance Company, a Connecticut
                                     corporation

                                     By:    /s/  Peter S. Atwood
                                        ----------------------------------------
                                     Name:  Peter S. Atwood
                                          --------------------------------------
                                     Title: Managing Director
                                           -------------------------------------

                                     PURCHASER:

                                     Columbia Equity LP, a Virginia limited
                                     partnership

                                     By: Columbia Equity Trust, Inc., a Maryland
                                         corporation, its general partner

                                         By:    /s/  Oliver T. Carr, III
                                            ------------------------------------
                                         Name:  Oliver T. Carr, III
                                              ----------------------------------
                                         Title: Chairman and CEO
                                               ---------------------------------

                                      -12-
<PAGE>

                                    EXHIBIT A

                                   ASSIGNMENT

Aetna Life Insurance Company, a Connecticut corporation ("Assignor"), for good
and valuable consideration paid to the Assignor by Columbia Equity, LP, a
[___________________] ("Assignee"), pursuant to the Membership Interest Sale
Agreement dated as of ____________, 2005, by and between Assignor and Assignee
(the "Agreement") and for other good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, does hereby sell, assign, transfer,
convey and deliver to the Assignee, its successors and assigns, without
representation or warranty except as set forth in the Agreement, good and
indefeasible title to the Membership Interest, free and clear of all liens,
encumbrances, security interests, prior assignments, voting agreements,
conditions, restrictions, pledges, claims, and other matters affecting title
thereto, subject to the LLC Operating Agreement. Assignee does hereby accept the
foregoing Assignment and assumes and agrees to be responsible for all
liabilities and obligations under the LLC Operating Agreement from and after the
date hereof relating to the Membership Interest.

Capitalized terms used but not defined herein shall have the respective meanings
ascribed to them in the Agreement.

IN WITNESS WHEREOF, the parties hereto have caused this Assignment and
Assumption Agreement to be signed by a duly authorized officer of each, this __
day of _____, 2005.

                                    ASSIGNOR:

                                                           , a
                                    -----------------------

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

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

                                    ASSIGNEE:

                                                           , a
                                    -----------------------

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

                                    By:
                                        ----------------------------------------
                                    Name:
                                          --------------------------------------
                                    Title:
                                           -------------------------------------<PAGE>

                                                                   EXHIBIT 10.35

                       MEMBERSHIP INTEREST SALE AGREEMENT
                                 (MADISON PLACE)

                                 BY AND BETWEEN

                          AETNA LIFE INSURANCE COMPANY,
                            A CONNECTICUT CORPORATION

                                    AS SELLER

                                       AND

                               COLUMBIA EQUITY LP,
                         A VIRGINIA LIMITED PARTNERSHIP,

                                  AS PURCHASER

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<S>                                                                                                            <C>
ARTICLE I  THE SALE...............................................................................................1

         1.1        Sale of Membership Interest...................................................................1
         1.2        Purchase Price................................................................................1

ARTICLE II  REPRESENTATIONS AND COVENANTS.........................................................................2

         2.1        Representations by Purchaser..................................................................2
         2.2        Representations by Seller.....................................................................3
         2.3        Covenants of Purchaser........................................................................4
         2.4        Covenants of Seller...........................................................................4

ARTICLE III  Conditions Precedent to the Closing..................................................................5

         3.1        Conditions to Purchaser's Obligations.........................................................5
         3.2        Conditions to Seller's Obligations............................................................6

ARTICLE IV  Closing and Closing Documents.........................................................................7

         4.1        Closing.......................................................................................7
         4.2        Seller's Deliveries...........................................................................7
         4.3        Purchaser's Deliveries........................................................................7
         4.4        Fees and Expenses; Closing Costs..............................................................8
         4.5        Distributions Prior to Closing................................................................8

ARTICLE V  Miscellaneous..........................................................................................8

         5.1        Notices.......................................................................................8
         5.2        Entire Agreement; Modifications and Waivers; Cumulative Remedies.............................10
         5.3        Exhibits.....................................................................................10
         5.4        Successors and Assigns.......................................................................10
         5.5        Article Headings.............................................................................10
         5.6        Governing Law................................................................................10
         5.7        Counterparts.................................................................................10
         5.8        Survival.....................................................................................10
         5.9        Severability.................................................................................10
         5.10       Attorneys' Fees..............................................................................11

EXHIBITS
         A          Assignment and Assumption Agreement
         B          First Amendment to Operating Agreement of Madison Place, LLC
</TABLE>

<PAGE>

                       MEMBERSHIP INTEREST SALE AGREEMENT

        THIS MEMBERSHIP INTEREST SALE AGREEMENT (this "Agreement") is made as of
this 25th day of May, 2005 by and between Aetna Life Insurance Company, a
Connecticut corporation ("Seller"); and Columbia Equity, LP, a Virginia limited
partnership ("Purchaser").

                                    RECITALS

        A.      Madison Place LLC, a Virginia limited liability company (the
"LLC") is the owner of certain land located at 500 Montgomery Street,
Alexandria, Virginia (the "Land") and the office building and related
improvements located thereon (the "Improvements"), which Land and Improvements
(collectively, the "Property") are more commonly known as Madison Place.

        B.      Seller is the record and beneficial owner of Eighty-Five percent
(85%) ("Seller's Share") of the membership interests in the LLC.

        C.      Seller desires to sell a Thirty-Five percent (35%) membership
interest in the LLC (the "Membership Interest") to Purchaser, on the terms and
conditions hereinafter set forth.

        D.      Purchaser desires to purchase the Membership Interest from
Seller, on the terms and conditions hereinafter set forth.

                                    AGREEMENT

        NOW, THEREFORE, for and in consideration of the mutual covenants herein
contained, the parties hereto agree as follows:

                                   ARTICLE I
                                    THE SALE

        1.1     Sale of Membership Interest. Seller agrees to sell, transfer,
assign and convey the Membership Interest to Purchaser, and Purchaser agrees to
purchase and accept transfer of the Membership Interest pursuant to the terms
and conditions set forth in this Agreement. The Membership Interest shall be
transferred to Purchaser free and clear of all liens, encumbrances, security
interests, prior assignments or conveyances, conditions, restrictions, voting
agreements, claims, and any other matters affecting title thereto (other than
the LLC's operating agreement (the "LLC Operating Agreement")).

        1.2     Purchase Price. The purchase price (the "Purchase Price") for
which Seller agrees to sell and assign the Membership Interest to Purchaser, and
which Purchaser agrees to pay to Seller, subject to the terms of this Agreement,
shall be Four Million Fifty Thousand Dollars ($4,050,000).

                                      -1-
<PAGE>

                                   ARTICLE II
                          REPRESENTATIONS AND COVENANTS

        2.1     Representations by Purchaser. Purchaser hereby represents and
warrants to Seller that the following statements are true, correct, and complete
in every material respect as of the date of this Agreement and will be true,
correct, and complete as of the Closing Date:

                (a)     Organization and Power. Purchaser is duly organized and
validly existing as a limited partnership under the laws of the Commonwealth of
Virginia, and has full right, power, and authority to enter into this Agreement
and to perform all of its obligations under this Agreement; and, the execution
and delivery of this Agreement and the performance by Purchaser of its
obligations under this Agreement have been duly authorized by all requisite
action of Purchaser and require no further action or approval of Purchaser's
partners or of any other individuals or entities in order to constitute this
Agreement as a binding and enforceable obligation of Purchaser.

                (b)     Noncontravention. Neither the entry into nor the
performance of, or compliance with, this Agreement by Purchaser has resulted, or
will result, in any violation of, or default under, or result in the
acceleration of, any obligation under the partnership agreement of Purchaser, or
any mortgage, indenture, lien agreement, note, contract, permit, judgment,
decree, order, restrictive covenant, statute, rule, or regulation applicable to
Purchaser.

                (c)     Litigation. To Seller's actual knowledge, there is no
action, suit, or proceeding, pending or known to be threatened, against or
affecting Purchaser in any court or before any arbitrator or before any federal,
state, municipal, or other governmental department, commission, board, bureau,
agency or instrumentality which (i) in any manner raises any question affecting
the validity or enforceability of this Agreement, (ii) would reasonably be
expected to materially and adversely affect the business, financial position, or
results of operations of Purchaser, (iii) would reasonably be expected to
materially and adversely affect the ability of Purchaser to perform its
obligations hereunder, or under any document to be delivered pursuant hereto.

                (d)     Consents. Each consent, approval, authorization, order,
license, certificate, permit, registration, designation, or filing by or with
any governmental agency or body necessary for the execution, delivery, and
performance of this Agreement or the transactions contemplated hereby by
Purchaser has been obtained.

                (e)     Bankruptcy with respect to Purchaser. No Act of
Bankruptcy has occurred with respect to Purchaser. As used herein, "Act of
Bankruptcy" shall mean if a party hereto shall (A) apply for or consent to the
appointment of, or the taking of possession by, a receiver, custodian, trustee
or liquidator of itself or of all or a substantial part of its property, (B)
admit in writing its inability to pay its debts as they become due, (C) make a
general assignment for the benefit of its creditors, (D) file a voluntary
petition or commence a voluntary case or proceeding under the Federal Bankruptcy
Code (as now or hereafter in effect), (E) be adjudicated bankrupt or insolvent,
(F) file a petition seeking to take advantage of any other law relating to
bankruptcy, insolvency, reorganization, winding-up or composition or adjustment
of debts, (G) fail to controvert in a timely and appropriate manner, or
acquiesce in writing to, any petition filed

                                      -2-
<PAGE>

against it in an involuntary case or proceeding under the Federal Bankruptcy
Code (as now or hereafter in effect), or (H) take any action for the purpose of
effecting any of the foregoing.

                (f)     Brokerage Commission. Purchaser has not engaged the
services of, nor has it or will it or Seller become liable to, any real estate
agent, broker, finder or any other person or entity for any brokerage or
finder's fee, commission or other amount with respect to the transactions
described herein on account of any action by Purchaser. Purchaser hereby agrees
to indemnify and hold Seller and its employees, directors, members, partners,
affiliates and agents harmless against any claims, liabilities, damages or
expenses arising out of a breach of the foregoing. This indemnification shall
survive Closing or any termination of this Agreement.

        2.2     Representations by Seller. Seller hereby represents and warrants
unto Purchaser that each and every one of the following statements is true,
correct, and complete in every material respect as of the date of this Agreement
and will be true, correct, and complete as of the Closing Date:

                (a)     Organization and Power. Seller is duly organized,
validly existing, and in good standing as a corporation under the laws of the
State of Connecticut. Seller has full right, power, and authority to enter into
this Agreement and to perform all of its obligations under this Agreement; and
the execution and delivery of this Agreement and the performance by Seller of
its obligations hereunder have been duly authorized by all requisite action of
Seller and require no further action or approval of Seller's members or managers
or of any other individuals or entities in order to constitute this Agreement as
a binding and enforceable obligation of Seller.

                (b)     Noncontravention. Neither the entry into nor the
performance of, or compliance with, this Agreement by Seller has resulted, or
will result, in any violation of, or default under, or result in the
acceleration of, any obligation under any limited liability company agreement,
operating agreement, regulation, mortgage, indenture, lien agreement, note,
contract, permit, judgment, decree, order, restrictive covenant, statute, rule,
or regulation applicable to Seller or to the Membership Interest.

                (c)     Litigation. There is no action, suit, claim, or
proceeding pending against or threatened against or affecting Seller or the
Membership Interest in any court, or before any arbitrator, or before any
federal, state, municipal or other governmental department, commission, board,
bureau, agency or instrumentality which (A) in any manner raises any question
affecting the validity or enforceability of this Agreement, (B) would reasonably
be expected to materially and adversely affect the business, financial position
or results of operations of Seller, (C) would reasonably be expected to
materially and adversely affect the ability of Seller to perform its obligations
hereunder, or under any document to be delivered pursuant hereto, (D) would
reasonably be expected to create a lien on the Membership Interest, any part
thereof, or any interest therein, or (E) would reasonably be expected to
adversely affect the Membership Interest, any part thereof, or any interest
therein.

                (d)     Good Title. (A) Seller has good title on the date hereof
and will have good title on the Closing Date to the Membership Interest (other
than the LLC Operating Agreement), (B) the Membership Interest on the date
hereof is and on the Closing Date will be free and clear of all liens,
encumbrances, pledges, voting agreements and security interests

                                      -3-
<PAGE>

whatsoever (other than the LLC Operating Agreement), and (C) Seller has not
granted any other person or entity an option to purchase or a right of first
refusal upon the Membership Interest nor are there any agreements or
understandings between Seller and any other person or entity with respect to the
disposition of the Membership Interest (other than the LLC Operating Agreement).

                (e)     No Consents. Each consent, approval, authorization,
order, license, certificate, permit, registration, designation, or filing by or
with, any governmental agency or body necessary of the execution, delivery, and
performance of this Agreement or the transactions contemplated hereby by Seller
has been obtained or will be obtained on or before the Closing Date.

                (f)     Bankruptcy with respect to Seller. No Act of Bankruptcy
has occurred with respect to Seller.

                (g)     Brokerage Commission. Seller has not engaged the
services of, nor has it or will it or Purchaser become liable to, any real
estate agent, broker, finder or any other person or entity for any brokerage or
finder's fee, commission or other amount with respect to the transactions
described herein on account of any action by Seller. Seller hereby agrees to
indemnify and hold Purchaser and its employees, directors, members, partners,
affiliates and agents harmless against any claims, liabilities, damages or
expenses arising out of a breach of the foregoing. This indemnification shall
survive Closing or any termination of this Agreement.

                (h)     Default. To Seller's actual knowledge, Seller is not in
default under the LLC Operating Agreement.

        2.3     Covenants of Purchaser. Purchaser agrees as follows:

                (a)     Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Purchaser shall perform, execute, and deliver
or cause to be performed, executed, and delivered at the Closing or after the
Closing, any and all further acts, instruments, and agreements and provide such
further assurances as Seller may reasonably require to consummate the
transactions contemplated hereunder.

        2.4     Covenants of Seller. Seller agrees as follows:

                (a)     Actions Regarding Membership Interest. Except as
otherwise permitted hereby, from the date hereof until the Closing Date, Seller
shall use reasonable commercial efforts not to take any action or fail to take
any action reasonably within Seller's control the result of which would (1) have
a material adverse effect on the Membership Interest, the Property, Seller's
ability to sell, transfer, assign and convey the Membership Interest to
Purchaser or Purchaser's ability to continue the ownership thereof after the
Closing Date or (2) cause any of the representations and warranties contained in
Section 2.2 to be untrue as of the Closing Date.

                (b)     Confidentiality. Seller acknowledges that matters
relating to the Columbia Equity Trust, Inc. (the "REIT"), the initial public
offering of REIT securities (the "IPO") and this Agreement (collectively, the
"Information") are confidential in nature. Therefore, Seller

                                      -4-
<PAGE>

covenants and agrees to keep the Information confidential and will not (except
as required by applicable law, regulation or legal process, and only after
compliance with the provisions of this Section 2.4) prior to the IPO, without
Purchaser's prior written consent, disclose any Information in any manner
whatsoever; provided, however, that the Information may be revealed only to
Seller's employees, legal counsel and financial advisors, each of whom shall be
informed of the confidential nature of the Information. In the event that Seller
or its key employees, legal counsel or financial advisors (collectively, the
"Information Group") are requested pursuant to, or required by, applicable law,
regulation or legal process to disclose any of the Information, the applicable
member of the Information Group will notify Purchaser promptly so that it may
seek a protective order or other appropriate remedy or, in its sole discretion,
waive compliance with the terms of this Section 2.4. Seller acknowledges that
remedies at law may be inadequate to protect Purchaser or the REIT against any
actual or threatened breach of this Section 2.4, and, without prejudice to any
other rights and remedies otherwise available, Seller agrees to the granting of
injunctive relief in favor of the REIT and/or Purchaser without proof of actual
damages.

                (c)     Other Members' Transactions. Seller hereby consents to
the assignment by each of the other members of the LLC of such other members'
membership interest in the LLC to Purchaser and agrees to the admission of
Purchaser as a member in the LLC.

                (d)     Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Seller shall perform, execute, and deliver or
cause to be performed, executed, and delivered at the Closing or after the
Closing, any and all further acts, instruments, and agreements and provide such
further assurances as Purchaser may reasonably require to consummate the
transactions contemplated hereunder.

                                  ARTICLE III
                      CONDITIONS PRECEDENT TO THE CLOSING

        3.1     Conditions to Purchaser's Obligations. In addition to any other
conditions set forth in this Agreement, Purchaser's obligation to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.1, all of which shall be
conditions precedent to Purchaser's obligations under this Agreement.

                (a)     Seller's Obligations. Seller shall have performed all
obligations of Seller hereunder which are to be performed prior to Closing, and
shall have delivered or caused to be delivered to Purchaser, all of the
documents and other information required of Seller pursuant to Section 4.2.

                (b)     Seller's Representations and Warranties. Seller's
representations and warranties set forth in Section 2.2 shall be true and
correct in all material respects as if made again on the Closing Date, and
Seller shall have executed and delivered to Purchaser at Closing a certificate
to the foregoing effect.

                                      -5-
<PAGE>

                (c)     No Injunction. On the Closing Date, there shall be no
effective injunction, writ, preliminary restraining order or other order issued
by a court of competent jurisdiction restraining or prohibiting the consummation
of the transactions contemplated hereby.

                (d)     Completion of IPO. The IPO shall have been completed.

                (e)     Other Member's Consent. The members of Carr Capital
Madison, LLC shall have sold their interests in Carr Capital Madison, LLC to
Purchaser or consented to the Amended LLC Operating Agreement (as hereinafter
defined) and to the transactions contemplated by this Agreement.

                (f)     Mortgage Lender Consent. The holder of the mortgage loan
secured by the Property shall have consented to the transactions contemplated by
this Agreement.

                (g)     Purchaser's General Partner. The REIT shall be the sole
general partner of the Purchaser and have control over the management and
operations of the Purchaser.

                (h)     Closing. The Closing shall have occurred on or prior to
June 30, 2005.

        3.2     Conditions to Seller's Obligations. In addition to any other
conditions set forth in this Agreement, Seller's obligations to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.2, all of which shall be
conditions precedent to Seller's obligations under this Agreement.

                (a)     Purchaser's Obligations. Purchaser shall have performed
all obligations of Purchaser hereunder which are to be performed prior to
Closing, and shall have delivered or caused to be delivered to Seller, all of
the documents and other information required of Purchaser pursuant to Section
4.3.

                (b)     Purchaser's Representations and Warranties. Purchaser's
representations and warranties set forth in Section 2.1 shall be true and
correct in all material respects as if made again on the Closing Date, and
Purchaser shall have executed and delivered to Seller at Closing a certificate
to the foregoing effect.

                (c)     No Injunction. On the Closing Date, there shall be no
effective injunction, writ, preliminary restraining order or other order issued
by a court of competent jurisdiction restraining or prohibiting the consummation
of the transactions contemplated hereby.

                (d)     Completion of IPO. The IPO shall have been completed.

                (e)     Other Member's Consent. The members of Carr Capital
Madison, LLC shall have sold their interests in Carr Capital Madison, LLC to
Purchaser or consented to the Amended LLC Operating Agreement (as hereinafter
defined) and to the transactions contemplated by this Agreement.

                (f)     Mortgage Lender Consent. The holder of the mortgage loan
secured by the Property shall have consented to the transactions contemplated by
this Agreement.

                                      -6-
<PAGE>

                (g)     Purchaser's General Partner. The REIT shall be the sole
general partner of the Purchaser and have control over the management and
operations of the Purchaser.

                (h)     Closing. The Closing shall have occurred on or prior to
June 30, 2005.

                                   ARTICLE IV
                          CLOSING AND CLOSING DOCUMENTS

        4.1     Closing. The consummation and closing (the "Closing") of the
transactions contemplated under this Agreement shall take place at the offices
of Hunton & Williams LLP, Washington, D.C., or such other place as is mutually
agreeable to the parties, on the date of the closing of the IPO (the "Closing
Date"), or as otherwise set by agreement of the parties; provided, however, that
this Agreement shall terminate if Closing does not occur prior to June 30, 2005.

        4.2     Seller's Deliveries. At the Closing, Seller shall deliver the
following to Purchaser in addition to all other items required to be delivered
to Purchaser by Seller:

                (a)     Assignment of Membership Interest. The Assignment and
Assumption Agreement, in substantially the form of Exhibit A attached hereto
(the "Assignment and Assumption Agreement") executed by Seller.

                (b)     Amended LLC Operating Agreement. The Amended and
Restated Operating Agreement of the LLC, in substantially the form of Exhibit B
attached hereto (the "Amended LLC Operating Agreement") executed by Seller.

                (c)     Authority Documents. Evidence satisfactory to Purchaser
that the person or persons executing the closing documents on behalf of Seller
has full right, power, and authority to do so.

                (d)     FIRPTA Certificate. An affidavit from Seller certifying
pursuant to Section 1445 of the Internal Revenue Code that Seller is not a
foreign corporation, foreign partnership, foreign trust, foreign estate or
foreign person (as those terms are defined in the Internal Revenue Code and the
Income Tax Regulations promulgated thereunder), in form and substance
satisfactory to Purchaser.

                (e)     Certificate of Representations and Warranties. The
certificate required by Section 3.1(b).

                (f)     Other Documents. Any other document or instrument
reasonably requested by Purchaser or required hereby.

        4.3     Purchaser's Deliveries. At the Closing, Purchaser shall deliver
the following to Seller:

                (a)     Purchase Price. The Purchase Price.

                                      -7-
<PAGE>

                (b)     Assumption of Membership Interest. The Assignment and
Assumption Agreement executed by Purchaser (and any other members thereof other
than Seller).

                (c)     Amended LLC Operating Agreement. The Amended LLC
Operating Agreement executed by Purchaser.

                (d)     Authority Documents. Evidence satisfactory to Seller
that the person or persons executing the closing documents on behalf of
Purchaser have full right, power, and authority to do so.

                (e)     Certificate of Representations and Warranties. The
certificate required by Section 3.2.

                (f)     Other Documents. Any other document or instrument
reasonably requested by Seller or required hereby.

        4.4     Fees and Expenses; Closing Costs. Purchaser shall pay all fees,
expenses and closing costs relating to the transactions contemplated by this
Agreement; provided however, that Seller shall pay its own attorneys' and
consultants' fees and expenses.

        4.5     Distributions Prior to Closing. Purchaser shall pay funds in the
amount of the Purchase Price to Seller without any setoffs or adjustments.
Purchaser and Seller agree that, after the date hereof and through the Closing
Date, the LLC will continue to be operated in the normal course of business
consistent with the LLC Operating Agreement and the Approved Budget (as defined
in the LLC Operating Agreement) for 2005. On or before Closing, cash held by or
for the benefit of the LLC (excluding lender-held reserves in excess of funds
due to the LLC for capital costs incurred prior to Closing) in excess of
$900,000 shall be distributed to the LLC's members in accordance with the
provisions of the LLC Operating Agreement.

                                   ARTICLE V
                                  MISCELLANEOUS

        5.1     Notices. Any notice provided for by this Agreement and any other
notice, demand, or communication required hereunder shall be in writing and
either delivered in person (including by confirmed facsimile transmission) or
sent by registered or certified mail or overnight courier, return receipt
requested, in a sealed envelope, postage prepaid, and addressed to the party for
which such notice, demand or communication is intended at such party's address
as set forth in this Section. All notices to Purchaser shall be addressed as
follows:

                                      -8-
<PAGE>

        PURCHASER:

                Columbia Equity LP
                c/o Carr Capital Corporation
                1750 H Street, NW, Suite 500
                Washington, DC 20005
                Telephone: (202) 303-3060
                Facsimile: (202) 303-3078
                Attn: Oliver T. Carr, III

        WITH COPIES TO:

                Hunton & Williams LLP
                1900 K Street, Suite 1200
                Washington, DC 20006
                Telephone: (202) 778-2221
                Facsimile: (202) 778-2201
                Attn: John M. Ratino

Seller's address for all purposes under this Agreement shall be the following:

        SELLER:

                Aetna Life Insurance Company
                151 Farmington Avenue, RT 11
                Hartford, Connecticut 06156-9642
                Telephone: (860) 273-1781
                Facsimile: (860) 273-1620
                Attn: Peter S. Atwood

        WITH COPIES TO:

                Bingham McCutchen LLP
                One State Street
                Hartford, Connecticut 06103
                Telephone: (860) 240-2759
                Facsimile: (860) 240-2800
                Attn: R. Jeffrey Smith, Esq.

                Any address or name specified above may be changed by a notice
                given by the addressee to the other party. Any notice, demand or
                other communication shall be deemed given and effective as of
                the date of delivery in person or receipt set forth on the
                return receipt. The inability to deliver because of changed
                address of which no notice was given, or rejection or other
                refusal to accept any notice, demand or other communication,
                shall be deemed to be receipt of the notice, demand or other
                communication as of the date of such attempt to deliver or
                rejection or refusal to accept.

                                      -9-
<PAGE>

        5.2     Entire Agreement; Modifications and Waivers; Cumulative
Remedies. This Agreement supersedes any existing letter of intent between the
parties, constitutes the entire agreement among the parties hereto and may not
be modified or amended except by instrument in writing signed by the parties
hereto, and no provisions or conditions may be waived other than by a writing
signed by the party waiving such provisions or conditions. No delay or omission
in the exercise of any right or remedy accruing to Seller or Purchaser upon any
breach under this Agreement shall impair such right or remedy or be construed as
a waiver of any such breach theretofore or thereafter occurring. The waiver by
Seller or Purchaser of any breach of any term, covenant, or condition herein
stated shall not be deemed to be a waiver of any other breach, or of a
subsequent breach of the same or any other term, covenant, or condition herein
contained. All rights, powers, options, or remedies afforded to Seller or
Purchaser either hereunder or by law shall be cumulative and not alternative,
and the exercise of one right, power, option, or remedy shall not bar other
rights, powers, options, or remedies allowed herein or by law, unless expressly
provided to the contrary herein.

        5.3     Exhibits. All exhibits referred to in this Agreement and
attached hereto are hereby incorporated in this Agreement by reference.

        5.4     Successors and Assigns. Except as set forth in this Article,
this Agreement may not be assigned by Purchaser or Seller without the prior
approval of the other party hereto. This Agreement shall be binding upon, and
inure to the benefit of, Seller, Purchaser, and their respective legal
representatives, successors, and permitted assigns.

        5.5     Article Headings. Article headings and article and section
numbers are inserted herein only as a matter of convenience and in no way
define, limit, or prescribe the scope or intent of this Agreement or any part
hereof and shall not be considered in interpreting or construing this Agreement.

        5.6     Governing Law. This Agreement shall be construed and interpreted
in accordance with the laws of the Commonwealth of Virginia, without regard to
conflicts of laws principles.

        5.7     Counterparts. This Agreement may be executed in any number of
counterparts and by any party hereto on a separate counterpart, each of which
when so executed and delivered shall be deemed an original and all of which
taken together shall constitute but one and the same instrument.

        5.8     Survival. All representations and warranties contained in this
Agreement, and all covenants and agreements contained in the Agreement which
contemplate performance after the Closing Date shall survive the Closing.

        5.9     Severability. In case any one or more of the provisions
contained in this Agreement shall for any reason be held to be invalid, illegal,
or unenforceable in any respect, such invalidity, illegality, or
unenforceability shall not affect any other provision hereof, and this Agreement
shall be construed as if such invalid, illegal, or unenforceable provision had
never been contained herein.

                                      -10-
<PAGE>

        5.10    Attorneys' Fees. Should a party employ an attorney or attorneys
to enforce any of the provisions hereof or to protect its interest in any manner
arising under this Agreement, or to recover damages for breach of this
Agreement, any non-prevailing party in any action pursued in a court of
competent jurisdiction (the finality of which is not legally contested) shall
pay to the prevailing party all reasonable costs, damages, and expenses,
including reasonable attorneys' fees, expended or incurred in connection
therewith.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK.]
                     [SIGNATURES APPEAR ON FOLLOWING PAGES.]

                                      -11-
<PAGE>

        IN WITNESS WHEREOF, this Agreement has been entered into effective as of
the date first above written.

                                 SELLER:

                                 Aetna Life Insurance Company, a Connecticut
                                 corporation

                                 By:    /s/  Peter S. Atwood
                                    --------------------------------------------
                                 Name:  Peter S. Atwood
                                      ------------------------------------------
                                 Title: Managing Director
                                       -----------------------------------------

                                 PURCHASER:

                                 Columbia Equity LP, a Virginia limited
                                 partnership

                                 By: Columbia Equity Trust, Inc., a Maryland
                                     corporation, its general partner

                                     By: /s/  Oliver T. Carr, III
                                        ----------------------------------------
                                     Name: Oliver T. Carr, III
                                     Title: Chairman and Chief Executive Officer

                                      -12-
<PAGE>

                                    EXHIBIT A

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

Aetna Life Insurance Company, a Connecticut corporation ("Assignor"), for good
and valuable consideration paid to the Assignor by Columbia Equity, LP, a
[___________________] ("Assignee"), pursuant to the Membership Interest Sale
Agreement dated as of ____________, 2005 by and between Assignor and Assignee
(the "Agreement") and for other good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, does hereby sell, assign, transfer,
convey and deliver to the Assignee, its successors and assigns, without
representation or warranty except as set forth in the Agreement, good and
indefeasible title to the Membership Interest, free and clear of all liens,
encumbrances, security interests, prior assignments, voting agreements,
conditions, restrictions, pledges, claims, and other matters affecting title
thereto, subject to the LLC Operating Agreement. Assignee does hereby accept the
foregoing Assignment and assumes and agrees to be responsible for all
liabilities and obligations under the LLC Operating Agreement from and after the
date hereof relating to the Membership Interest.

Capitalized terms used but not defined herein shall have the respective meanings
ascribed to them in the Agreement.

IN WITNESS WHEREOF, the parties hereto have caused this Assignment and
Assumption Agreement to be signed by a duly authorized officer of each, this __
day of _____, 2005.

                                     ASSIGNOR:

                                                            , a
                                     -----------------------

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

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

                                     ASSIGNEE:

                                                            , a
                                     -----------------------

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

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

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