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                                                                   EXHIBIT 10.21
                       ACKNOWLEDGMENT AND WAIVER AGREEMENT

         This Agreement ("AGREEMENT"), dated December ___, 2005, is entered into
by and between Prentiss Properties Trust, a Maryland real estate investment
trust (the "COMPANY") and ____________ ("KEY EMPLOYEE").

      1. Acknowledgement. Pursuant to the terms of the Prentiss Properties Trust
         Change in Control Severance Protection Plan for Key Employees (the
         "PLAN"), Key Employee is eligible to receive benefits set forth in
         Section 4.2 of the Plan upon a Qualifying Termination (as it is defined
         in the Plan) of employment.

      2. Waiver of Benefits. Key Employee hereby waives any and all rights which
         Key Employee has pursuant to the Plan in consideration for the payment
         of, prior to January 1, 2006, (i) a lump sum amount equal to the amount
         which Key Employee would have been eligible to receive under Section
         4.2(b) of the Plan, plus (ii) a lump sum amount equal to the aggregate
         sum of all the premiums (both employer and employee portion of such
         premiums) that would be payable by the Company to provide for the
         benefits set forth in Section 4.2(c) of the Plan. The total payment
         amount is listed on Exhibit A of this Agreement.

      3. Acceleration of Certain Equity Grants. Notwithstanding the foregoing,
         if the Qualifying Termination takes place within three years of the
         date of grant of any award of restricted stock or any option in
         Brandywine Realty Trust ("BRANDYWINE") which was granted in connection
         with the merger of the Company with Brandywine (the "MERGER"), then the
         restrictions with respect to any such grant of restricted stock shall
         immediately lapse, and any such option granted shall become immediately
         vested and exercisable.

      4. Section 4999 Tax Gross-Up. Article VI of the Plan, relating to Sections
         280G and 4999 of the Internal Revenue Code of 1986, as amended (the
         "CODE"), shall continue to apply in full force and effect as set forth
         in the Plan.

      5. Section 409A Tax Gross-Up.

              (a) In the event it shall be determined that the payment of the
         amounts listed on Exhibit A to this Agreement in 2005 (the "2005
         Payments") is or will be subject to the excise tax imposed by Section
         409A of the Code or any interest or penalties with respect to such
         payment or excise tax (such excise tax, together with any such interest
         and penalties, are collectively referred to as the "Excise Tax"), then
         Key Employee shall be entitled to receive an additional payment (a
         "Gross-Up Payment") in an amount such that after payment by Key
         Employee of all taxes (including any interest or penalties imposed with
         respect to such taxes), including but not limited to, any income tax,
         employment tax or Excise tax, imposed upon the Gross Up Payment, Key
         Employee retains an amount of the Gross-Up Payment equal to the Excise
         Tax imposed upon the Total Payments. For purposes of determining the
         amount of the Gross-Up Payment, Executive shall be deemed to pay
         federal income tax and employment taxes at the highest marginal rate of
         federal income and employment taxation in the calendar year in which
         the Gross-Up Payment is to be made and state and local income taxes at
         the highest marginal rate of taxation in the state and locality of
         Executive's residence (or, if greater, the state and locality in which
         Executive is required to file a nonresident income tax return with
         respect to the Payment) on the date of termination, net of the maximum
         reduction in federal income taxes that may be obtained from the
         deduction of such state and local taxes.




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              (b) If it is finally determined that any of the Total Payments are
         subject to Excise Tax any determinations as to the amount of the
         Gross-Up Payment shall be made by an independent accounting firm
         selected by the Company (the "Accounting Firm"), which shall provide
         its determination (the "Determination"), together with detailed
         supporting calculations regarding the amount of any Gross-Up Payment
         and any other relevant matter, both to the Company and Key Employee by
         no later than ten (10) days following such final determination, or such
         earlier time as is requested by the Company or Key Employee (if Key
         Employee reasonably believes that any of the Total Payments may be
         subject to the Excise Tax). If a Gross-Up Payment is determined to be
         payable, it shall be paid to Key Employee within twenty (20) days after
         the Determination (and all accompanying calculations and other material
         supporting the Determination) is delivered to the Company by the
         Accounting Firm. Any determination by the Accounting Firm shall be
         binding upon the Company and Key Employee, absent manifest error.

              (c) If a claim by a federal, state or local taxing authority is
         made against Key Employee, and if Key Employee intends to seek a
         Gross-Up Payment with respect thereto under this Section, Key Employee
         shall promptly notify (i) the Company in writing of such claims,
         setting forth such claims in reasonable detail and providing copies of
         any written documentation provided by the taxing authority in
         connection with its claims. The Company shall have fifteen (15) days
         after receipt of such notice to undertake, conduct and control, through
         counsel of its own choosing and at its own expense, the settlement or
         defense thereof, and Key Employee shall cooperate with it in connection
         therewith; provided, that Key Employee may participate in such
         settlement or defense through counsel chosen by Key Employee and paid
         at his own expense. So long as the Company is reasonably contesting any
         such claim in good faith, Key Employee shall not pay or settle any such
         claim without the consent of the Company, which consent shall not be
         unreasonably withheld. If the Company does not notify Key Employee in
         writing within fifteen (15) days after receipt of the Company's written
         notice of a claim to a Gross-Up Payment hereunder that it elects to
         undertake the defense thereof, Key Employee shall have the right to
         undertake, at the Company's cost, risk and expense, the defense,
         compromise or settlement of the claim, but shall not thereby waive any
         right to a Gross-Up Payment therefore pursuant to this Agreement. The
         Company shall pay Key Employee's expenses as and when incurred.

      6. Applicable Law. This Acknowledgment and Waiver will be governed and
         construed in accordance with the laws of the State of Texas.

      7. No Third-Party Beneficiaries. This Agreement is solely for the benefit
         of the parties to this Acknowledgment and Waiver and, except to the
         extent the Company is affected hereby, should not be deemed to confer
         upon third parties any remedy, claim, liability, reimbursement, claims
         or actions or other right in excess of those existing without reference
         to this Agreement.




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      8. Counterparts. This Agreement may be executed in one or more
         counterparts, each of which shall be deemed to be an original, but all
         of which together constitute one and the same instrument.

      9. Severability. Any provision of this Acknowledgment & Waiver that is
         prohibited or unenforceable in any jurisdiction shall, as to such
         jurisdiction, be ineffective to the extent of such prohibition or
         unenforceability without invalidating the remaining provisions hereof.
         Any such prohibition or unenforceability in any jurisdiction shall not
         invalidate or render unenforceable such provision in any other
         jurisdiction. Without prejudice to any rights or remedies otherwise
         available to any party to this Agreement, each party hereto
         acknowledges that the obligations of the parties hereto shall be
         specifically enforceable.

     10. Further Assurances. The parties hereto will execute and deliver or
         cause to be executed and delivered such further instruments and
         documents and will take such other actions as any other party to this
         Acknowledgment and Waiver may reasonably request in order to effectuate
         the purpose of this Acknowledgment and Waiver and to carry out the
         terms thereof.

     11. Successors to Company. This Acknowledgement and Waiver shall be binding
         upon and shall inure to the benefit of the Company, its successors and
         assigns and the Company shall require any successor or assign to
         expressly assume and agree to perform this Acknowledgment and Waiver in
         the same manner and to the same extent that the Company would be
         required to perform it if no such succession or assignment had taken
         place. The term "Company" as used herein shall mean a trust,
         corporation or other entity acquiring all or substantially all the
         assets and business of the Company whether by operation of law or
         otherwise.

     12. Assignability by Participant. Neither this Acknowledgment and Waiver
         nor any right or interest hereunder shall be assignable or transferable
         by Key Employee or his or her beneficiaries or legal representatives,
         except by will or by the laws of descent and distribution. This
         Acknowledgment and Waiver shall inure to the benefit of and be
         enforceable by a Key Employee's legal personal representative.

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         IN WITNESS WHEREOF, the parties hereto have caused this Acknowledgment
and Waiver to be duly executed as of the day and year first written above.





                                          KEY EMPLOYEE





                                          ______________________________________
                                          [Key Employee Name]



                                          THE COMPANY
                                          PRENTISS PROPERTIES TRUST





                                          By:   ________________________________
                                          Name: ________________________________
                                          Title: _______________________________