Exhibit 10.24

                             PS BUSINESS PARKS, L.P.

                        AMENDMENT TO AGREEMENT OF LIMITED
                             PARTNERSHIP RELATING TO
                      8 7/8% SERIES X CUMULATIVE REDEEMABLE
                                 PREFERRED UNITS

        This  Amendment to the Agreement of Limited  Partnership  of PS Business
Parks, L.P. a California limited  partnership (the  "Partnership"),  dated as of
the 7th day of  September,  1999  (this  "Amendment")  amends the  Agreement  of
Limited Partnership of the Partnership,  dated as of March 17, 1998 by and among
PS Business Parks, Inc. (the "General Partner") and each of the limited partners
executing a signature page thereto, as amended  (collectively,  the "Partnership
Agreement").  Capitalized terms used herein and not otherwise defined shall have
the  meanings  ascribed  to such  terms in the  Partnership  Agreement.  Section
references  are  (unless  otherwise  specified)  references  to sections in this
Amendment.

        WHEREAS,  pursuant to Section 4.2(a) of the Partnership  Agreement,  the
General Partner desires to cause the Partnership to issue  additional Units of a
new  class  and  series,  with  the  designations,   preferences  and  relative,
participating,  optional or other  special  rights,  powers and duties set forth
herein;

        WHEREAS,  pursuant to Section 4.2(a) of the Partnership  Agreement,  the
General  Partner,  without the consent of the  Limited  Partners,  may amend the
Partnership  Agreement by executing a written instrument setting forth the terms
of such amendment; and

        WHEREAS,  the General  Partner desires by this Amendment to so amend the
Partnership  Agreement  as of the date first set forth  above to provide for the
designation and issuance of such new class and series of Units.

        NOW,  THEREFORE,   the  Partnership   Agreement  is  hereby  amended  by
establishing  and fixing the rights,  limitations and preferences of a new class
and series of Units as follows:

        Section 1.  Definitions.  Capitalized terms not otherwise defined herein
shall have their  respective  meanings set forth in the  Partnership  Agreement.
Capitalized  terms that are used in this  Amendment  shall have the meanings set
forth below:

        (a)  "Liquidation  Preference"  means,  with  respect  to the  Series  X
Preferred  Units,  $25.00 per Series X  Preferred  Unit,  plus the amount of any
accumulated and unpaid Priority Return with respect to such unit, whether or not
declared,  minus any  distributions  in excess of the  Priority  Return that has
accrued with respect to such Series X Preferred Units to the date of payment.

        (b) "Parity  Preferred  Units" means any class or series of  Partnership
Interests of the Partnership now or hereafter authorized,  issued or outstanding
and expressly  designated by the Partnership to rank in parity with the Series X
Preferred  Units (as  hereinafter  defined)  with respect to  distributions  and
rights upon voluntary or involuntary  liquidation,  winding-up or dissolution of
the Partnership.  Notwithstanding  the differing  allocation rights set forth in
Section  4 below  that  apply to the  Series  A, B and C  Preferred  Units,  for
purposes  of this  Amendment  those  Series A, B and C  Preferred  Units and any
future  series of  preferred  units that rank in parity  with those  series also
shall be considered Parity Preferred Units to the Series X Preferred Units.

        (c)  "Priority  Return" means an amount equal to 8 7/8% per annum of the
Liquidation  Preference per Series X Preferred  Unit,  commencing on the date of
issuance of such Series X Preferred  Unit,  determined on the basis of a 365-day
year (and actual days for any period)  cumulative to the extent not  distributed
on any Series X Preferred Unit Distribution Payment Date.

        (d) "PTP" means a "publicly  traded  partnership"  within the meaning of
Section 7704 of the Code.

        Section 2.  Designation  and Number.  Pursuant to Section  4.2(a) of the
Partnership  Agreement,  a  series  of  Partnership  Units  in  the  Partnership
designated as the "8 7/8% Series X Cumulative  Redeemable  Preferred Units" (the
"Series X  Preferred  Units")  is  hereby  established.  The  number of Series X
Preferred  Units shall be  1,200,000.  The  Holders of Series X Preferred  Units
shall  not  have  any  Percentage  Interest  (as  such  term is  defined  in the
Partnership Agreement) in the Partnership.

      Section 3. Distributions.  (a) Payment of Distributions.  Subject to the
rights of holders of Parity Preferred Units as to the payment of  distributions,
pursuant  to  Section  5.1 of the  Partnership  Agreement,  holders  of Series X
Preferred  Units shall be entitled to receive,  when,  as and if declared by the
Partnership  acting  through the General  Partner,  the  Priority  Return.  Such
distributions  shall be  cumulative,  shall  accrue  from the  original  date of
issuance of the Series X Preferred Units and, notwithstanding Section 5.1 of the
Partnership  Agreement,  will be payable (i)  quarterly  in arrears on March 31,
June 30,  September 30 and December 31 of each year  commencing on September 30,
1999, and (ii) in the event of a redemption of Series X Preferred  Units (each a
"Series X  Preferred  Unit  Distribution  Payment  Date").  If any date on which
distributions  are to be made on the Series X Preferred  Units is not a Business
Day (as defined  herein),  then payment of the  distribution  to be made on such
date will be made on the Business Day  immediately  preceding such date with the
same  force and effect as if made on such  date.  Distributions  on the Series X
Preferred  Units will be made to the holders of record of the Series X Preferred
Units on the relevant record dates to be fixed by the Partnership acting through
the General  Partner,  which record dates shall in no event exceed  fifteen (15)
Business Days prior to the relevant Series X Preferred Unit Distribution Payment
Date (the "Series X Preferred Unit Partnership Record Date").

        (b) Prohibition on Distribution.  No distributions on Series X Preferred
Units  shall be  authorized  by the  General  Partner  or paid or set  apart for
payment by the  Partnership  at any such time as the terms and provisions of any
agreement of the  Partnership  or the General  Partner,  including any agreement
relating to their indebtedness, prohibits such authorization, payment or setting
apart for payment or provides that such authorization,  payment or setting apart
for payment would constitute a breach thereof or a default thereunder, or to the
extent that such  authorization  or payment shall be restricted or prohibited by
law.

        (c)  Distributions  Cumulative.  Distributions on the Series X Preferred
Units will accrue  whether or not the terms and  provisions  of any agreement of
the  Partnership,  including any agreement  relating to its  indebtedness at any
time  prohibit  the  current  payment  of  distributions,  whether  or  not  the
Partnership has earnings,  whether or not there are funds legally  available for
the  payment of such  distributions  and whether or not such  distributions  are
authorized.  Accrued but unpaid  distributions  on the Series X Preferred  Units
will accumulate as of the Series X Preferred Unit  Distribution  Payment Date on
which they first  become  payable.  Distributions  on account of arrears for any
past  distribution  periods  may be  declared  and  paid  at any  time,  without
reference  to a regular  Series X Preferred  Unit  Distribution  Payment Date to
holders of record of the Series X  Preferred  Units on the record  date fixed by
the  Partnership  acting through the General Partner which date shall not exceed
fifteen (15)  Business Days prior to the payment  date.  Accumulated  and unpaid
distributions will not bear interest.

        (d) Priority as to  Distributions.  Subject to the provisions of Article
13 of the Partnership Agreement:

        (i) so  long  as any  Series  X  Preferred  Units  are  outstanding,  no
distribution  of cash or other property shall be authorized,  declared,  paid or
set apart for payment on or with  respect to any class or series of  Partnership
Interest  ranking  junior as to the  payment of  distributions  or rights upon a
voluntary  or  involuntary   liquidation,   dissolution  or  winding-up  of  the
Partnership to the Series X Preferred Units (collectively,  "Junior Units"), nor
shall any cash or other  property  be set aside for or applied to the  purchase,
redemption  or other  acquisition  for  consideration  of any Series X Preferred
Units, any Parity Preferred Units or any Junior Units, unless, in each case, all
distributions  accumulated  on all Series X Preferred  Units and all classes and
series of  outstanding  Parity  Preferred  Units  have  been  paid in full.  The
foregoing sentence shall not prohibit (x) distributions payable solely in Junior
Units,  (y) the  conversion  of Junior  Units or  Parity  Preferred  Units  into
Partnership  Interests ranking junior to the Series X Preferred Units or (z) the
redemption of Partnership  Interests  corresponding to Series X Preferred Stock,
Parity  Preferred  Stock or Junior Stock to be purchased by the General  Partner
pursuant to the Articles of Incorporation  with respect to the General Partner's
common stock and comparable Articles of Incorporation provisions with respect to
other classes or series of capital stock of the General  Partner to preserve the
General Partner's status as a real estate  investment trust,  provided that such
redemption shall be upon the same terms as the  corresponding  purchase pursuant
to the Articles of Incorporation.

        (ii) So long as  distributions  have  not  been  paid in full  (or a sum
sufficient  for such full  payment  is not  irrevocably  deposited  in trust for
payment) upon the Series X Preferred  Units,  all  distributions  authorized and
declared  on the  Series  X  Preferred  Units  and  all  classes  or  series  of
outstanding  Parity Preferred Units shall be authorized and declared so that the
amount of distributions  authorized and declared per Series X Preferred Unit and
such other classes or series of Parity  Preferred  Units shall in all cases bear
to each other the same ratio that accrued  distributions  per Series X Preferred
Unit and such other classes or series of Parity Preferred Units (which shall not
include  any  accumulation  in  respect  of  unpaid   distributions   for  prior
distribution  periods if such class or series of Parity  Preferred  Units do not
have cumulative distribution rights) bear to each other.

    (e) No Further Rights.  Holders of Series X Preferred Units shall not be
entitled  to any  distributions,  whether  payable in cash,  other  property  or
otherwise in excess of the full cumulative distributions described herein.

        Section 4. Allocations.  Section 6.1(a)(ii) of the Partnership Agreement
is amended to read, in its entirety, as follows:

        "(ii) (A)  Notwithstanding  anything to the  contrary  contained in this
Agreement,  in any taxable year:  (1) the holders of Series A, B and C Preferred
Units shall first be  allocated  an amount of gross income equal to the Priority
Return  distributed to such holders in such taxable year, and (2) subject to any
prior  allocation of Profit pursuant to the loss chargeback set forth in Section
6.1(a)(ii)(B)  below,  the  holders of Series X  Preferred  Units  shall then be
allocated an amount of Profit equal to the Priority  Return  distributed to such
holders either in such taxable year or in prior taxable years to the extent that
such distributions have not previously been matched with an allocation of Profit
pursuant to this Section 6.1(a)(ii)(A)(2).

        (B) After the  Capital  Account  balances  of all  Partners  other  than
holders of any series of Preferred  Units have been  reduced to zero,  Losses of
the Partnership that otherwise would be allocated so as to cause deficit Capital
Account  balances for those other  Partners shall be allocated to the holders of
the Series A, B, C and X Preferred Units in proportion to the positive  balances
of their Capital Accounts until those Capital Account balances have been reduced
to zero. If Losses have been  allocated to the holders of the Series A, B, C and
X Preferred  Units  pursuant to the  preceding  sentence,  the first  subsequent
Profits  shall be  allocated  to those  preferred  partners so as to recoup,  in
reverse order, the effects of the loss allocations.

        (C) Upon  liquidation of the  Partnership or the interest of the holders
of Series A, B, C or X Preferred  Units in the  Partnership:  (1) items of gross
income or deduction shall first be allocated to the holders of Series A, B and C
Preferred Units in a manner such that,  immediately  prior to such  liquidation,
the Capital  Account  balances of such  holders  shall equal the amount of their
Liquidation  Preferences,  and (2) an amount of  Profit  or Loss  shall  then be
allocated  to the  holders of Series X  Preferred  Units in a manner  such that,
immediately  prior to such  liquidation,  the Capital  Account  balances of such
holders shall equal the amount of their Liquidation Preferences."

        Section 5. Optional Redemption. (a) Right of Optional Redemption. Except
as otherwise provided in this Amendment, the Series X Preferred Units may not be
redeemed prior to the fifth (5th)  anniversary of the issuance date. On or after
such date, the Partnership shall have the right to redeem the Series X Preferred
Units,  in whole (and not in part),  at any time, upon not less than 10 nor more
than 60 days written notice,  at a redemption  price,  payable in cash, equal to
the  Liquidation  Preference (the "Series X Redemption  Price").  The Redemption
Right  given to Limited  Partners in Section  8.6 of the  Partnership  Agreement
shall not be  available  to the holders of the Series X Preferred  Units and all
references  to Limited  Partners in said Section 8.6 (and related  provisions of
the Partnership  Agreement)  shall not include holders of the Series X Preferred
Units.  The Series X Redemption Price will not be payable out of proceeds from a
loan  obtained  by the  Partnership  solely  for the  purpose of payment of said
Series X Redemption Price.

        (b)  Procedures  for  Redemption.  (i) Notice of redemption  will be (A)
faxed,  and (B) mailed by the Partnership,  by certified mail,  postage prepaid,
not less than 10 nor more than 60 days prior to the redemption  date,  addressed
to the  respective  holders of record of the Series X  Preferred  Units at their
respective  addresses  as they  appear on the  records  of the  Partnership.  No
failure  to give or defect in such  notice  shall  affect  the  validity  of the
proceedings  for the redemption of any Series X Preferred Units except as to the
holder to whom such  notice was  defective  or not  given.  In  addition  to any
information  required by law each such notice  shall state:  (m) the  redemption
date, (n) the Redemption  Price,  (o) the aggregate number of Series X Preferred
Units to be redeemed,  (p) as provided in Section  5(b)(ii) below,  the place or
places where evidence of the surrender of such Series X Preferred Units shall be
delivered for payment of the Redemption  Price,  (q) that  distributions  on the
Series X  Preferred  Units to be  redeemed  will  cease  to  accumulate  on such
redemption  date and (r) that payment of the Redemption  Price will be made upon
presentation  of evidence of the  surrender of such Series X Preferred  Units as
set forth in Section 5(b)(ii) below.

        (ii) If the  Partnership  gives a notice of  redemption  in  respect  of
Series X Preferred Units (which notice will be irrevocable) then, by 12:00 noon,
New York City time, on the redemption  date, the  Partnership  will deliver into
escrow with an escrow agent acceptable to the Partnership and the holders of the
Series X  Preferred  Units (the  "Escrow  Agent")  the  Redemption  Price and an
executed  Redemption  Agreement,  in the form attached  hereto as Exhibit A (the
"Redemption   Agreement"),   and  an  Amendment  to  the  Agreement  of  Limited
Partnership evidencing the Redemption, in the form attached hereto as Exhibit B.
The holders of the Series X Preferred  Units shall also, by 12:00 noon, New York
City time, on the redemption date,  deliver into escrow with the Escrow Agent an
executed  Redemption  Agreement  and an executed  Amendment to the  Agreement of
Limited  Partnership  evidencing  the  Redemption.  Upon  delivery of all of the
above-described items by both parties, Escrow Agent shall release the Redemption
Price to the  holders of the  Series X  Preferred  Units and the  fully-executed
Redemption  Agreement and Amendment to Agreement of Limited  Partnership to both
parties.  On and  after  the date of  redemption,  distributions  will  cease to
accumulate  on the Series X Preferred  Units called for  redemption,  unless the
Partnership defaults in the payment thereof. If any date fixed for redemption of
Series X Preferred  Units is not a Business Day, then payment of the  Redemption
Price  payable  on such date will be made on the next  succeeding  day that is a
Business Day (and  without any interest or other  payment in respect of any such
delay) except that, if such Business Day falls in the next calendar  year,  such
payment will be made on the  immediately  preceding  Business  Day, in each case
with the same force and effect as if made on such date fixed for redemption.  If
payment of the Redemption  Price is improperly  withheld or refused and not paid
by the Partnership, distributions on such Series X Preferred Units will continue
to accumulate from the original redemption date to the date of payment, in which
case the actual  payment date will be considered  the date fixed for  redemption
for purposes of calculating the applicable Redemption Price.

        Section 6. Voting Rights. (a) General. Holders of the Series X Preferred
Units  will not have any  voting  rights  or  right  to  consent  to any  matter
requiring the consent or approval of the Limited  Partners,  except as set forth
in Section 14.1 of the Partnership  Agreement and in this Section 6. (Solely for
purposes of Section 14.1 of the Partnership  Agreement,  each Series X Preferred
Unit shall be treated as one Partnership Unit.)

        (b)  Certain  Voting  Rights.  So long as any Series X  Preferred  Units
remain  outstanding,  the Partnership shall not, without the affirmative vote of
the holders of at least a majority of the Series X Preferred  Units  outstanding
at the time:  (i)  authorize  or create,  or increase the  authorized  or issued
amount of, any class or series of  Partnership  Interests  ranking senior to the
Series X Preferred Units with respect to payment of distributions or rights upon
liquidation,  dissolution or winding-up or reclassify any Partnership  Interests
into  any  such  Partnership  Interest,  or  create,   authorize  or  issue  any
obligations or security convertible into or evidencing the right to purchase any
such Partnership Interests (for this purpose, partnership interests that rank in
parity  with  the  Series  A, B and C  Preferred  Units  or  other  series  with
equivalent  parity,  shall not be  treated  as  ranking  senior to, and shall be
treated as in parity  with,  the Series X Preferred  Units and any other  series
that rank in parity  with the  Series X  Preferred  Units);  (ii)  designate  or
create,  or increase the  authorized or issued  amount of, any Parity  Preferred
Units or reclassify  any authorized  Partnership  Interests into any such Parity
Preferred  Units,  or create,  authorize  or issue any  obligations  or security
convertible  into or evidencing the right to purchase any such shares,  but only
to the extent  such Parity  Preferred  Units are issued to an  Affiliate  of the
Partnership  on terms that differ from the terms of any Parity  Preferred  Units
issued to the public or  non-Affiliates of the Partnership (for purposes of this
Section 6(b)(ii),  an issuance to the General Partner shall not be treated as an
issuance to an Affiliate of the  Partnership  to the extent the issuance of such
Partnership  Interests was to allow the General  Partner to issue  corresponding
preferred stock to persons who are not Affiliates); or (iii) either (A) exchange
shares, consolidate, merge into or with, or convey, transfer or lease its assets
substantially  as an entirety to, any  corporation or other entity or (B) amend,
alter or repeal the provisions of the Partnership Agreement,  whether by merger,
consolidation  or otherwise,  that would  adversely  affect the powers,  special
rights, preferences,  privileges or voting power of the Series X Preferred Units
or the holders thereof;  provided,  however, that with respect to the occurrence
of a share  exchange,  merger,  consolidation  or a sale or  lease of all of the
Partnership's  assets  as an  entirety,  so long as (1) the  Partnership  is the
surviving  entity and the Series X Preferred Units remain  outstanding  with the
terms thereof unchanged, or (2) the resulting, surviving or transferee entity is
a partnership,  limited liability company or other pass-through entity organized
under the laws of any state and  substitutes  the Series X  Preferred  Units for
other interests in such entity having substantially the same terms and rights as
the Series X Preferred Units,  including with respect to  distributions,  voting
rights  and  rights  upon  liquidation,  dissolution  or  winding-up,  then  the
occurrence  of any such  event  shall  not be deemed to  adversely  affect  such
rights,  privileges  or voting  powers of the  holders of the Series X Preferred
Units;  and  provided  further  that any  increase in the amount of  Partnership
Interests  or the  creation  or  issuance  of  any  other  class  or  series  of
Partnership Interests, in each case ranking (y) junior to the Series X Preferred
Units with respect to payment of  distributions  or the  distribution  of assets
upon liquidation,  dissolution or winding-up, or (z) on a parity to the Series X
Preferred Units with respect to payment of  distributions or the distribution of
assets  upon  liquidation,   dissolution  or  winding-up,  to  the  extent  such
Partnership  Interests  are not issued to an  Affiliate of the  Partnership  (an
issuance  to the  General  Partner  shall not be  treated as an  issuance  to an
Affiliate  of the  Partnership  to the extent the  issuance of such  Partnership
Interests  was to allow the  General  Partner to issue  corresponding  preferred
stock to persons who are not Affiliates of the Partnership)  such issuance shall
not be deemed to adversely affect such rights, preferences, privileges or voting
powers. Notwithstanding anything to the contrary contained in this Section 6, if
holders  of a  majority  of the  Series X  Preferred  Units do not  approve of a
proposed action by the Partnership  described in clause (iii) of the immediately
preceding sentence which, in the reasonable judgment of the Partnership, results
in the holders of Series X Preferred Units having  substantially  the same terms
and  rights  as  the  Series  X  Preferred  Units,  including  with  respect  to
distributions,  voting  rights  and  rights  upon  liquidation,  dissolution  or
winding-up, and the holders of a majority of the Series X Preferred Units do not
affirmatively  vote in favor of such proposed  action,  then the Partnership may
proceed  with such  proposed  action and the sole  remedy of the  holders of the
Series X Preferred Units shall be the acceleration of the exchange date relating
to the Series X Preferred Units, as set forth in Section 8 of this Amendment. In
the  event  of  any  conflict  between  the  provisions  of  Section  4.2 of the
Partnership  Agreement and the  provisions of this Section 6, the  provisions of
this Section 6 shall control.

        Section 7. Transfer Restrictions.  (a) The holders of Series X Preferred
Units shall be subject to all of the provisions of Section 11 of the Partnership
Agreement  as modified by this  Section 7. Subject to the consent of the General
Partner,  which shall not be  unreasonably  withheld  or  delayed,  the Series X
Preferred Units may be transferred to a maximum of five (5) persons.  At no time
shall the number of holders of the Series X Preferred Units exceed five.

        (b)  Notwithstanding  anything to the contrary in Section  7(a),  if any
holder of Series X Preferred  Units  concludes  based upon  results or projected
results  that  there  exists  (in the  reasonable  judgment  of such  holder) an
imminent and  substantial  risk that such holder's  interest in the  Partnership
represents  or will  represent  more than 20% of the total  profits  or  capital
interests in the Partnership  for a taxable year  (determined in accordance with
Treasury  Regulations  Section  1.731-2,  then such holder shall be permitted to
transfer  so much of its  Series  X  Preferred  Units as may be  appropriate  to
alleviate the risk of not satisfying such 20% limit.

        Section  8.  Exchange  Rights.  (a)  Right  to  Exchange.  (i)  Series X
Preferred  Units will be  exchangeable in whole (and not in part) at any time on
or after the tenth (10th) anniversary of the date of issuance,  at the option of
the Partnership or a majority of the holders  thereof  (acting as a whole),  for
authorized  but  previously  unissued  shares  of 8  7/8%  Series  X  Cumulative
Redeemable  Preferred  Stock of the  General  Partner  (the  "Series X Preferred
Stock") at an  exchange  rate of one share of Series X  Preferred  Stock for one
Series X Preferred Unit, subject to adjustment as described below (the "Series X
Exchange  Price");  provided  that the  Series X  Preferred  Units  will  become
exchangeable  at any  time,  in whole  (and not in  part),  at the  option  of a
majority  of the  holders of Series X  Preferred  Units  (acting as a whole) for
Series X Preferred  Stock if (x) at any time full  distributions  shall not have
been timely made on any Series X  Preferred  Unit with  respect to six (6) prior
quarterly distribution periods, whether or not consecutive;  provided,  however,
that a distribution  in respect of Series X Preferred  Units shall be considered
timely made if made within two (2) Business Days after the  applicable  Series X
Preferred  Units  Distribution  Payment Date if at the time of such late payment
there shall not be any prior quarterly  distribution periods in respect of which
full distributions were not timely made, (y) upon receipt by a holder or holders
of Series X  Preferred  Units of (1) notice from the  General  Partner  that the
General  Partner or a Subsidiary  of the General  Partner has taken the position
that the  Partnership  is,  or upon the  occurrence  of a  defined  event in the
immediate  future  will  be, a PTP and (2) an  opinion  rendered  by an  outside
nationally  recognized  independent counsel familiar with such matters addressed
to a holder or holders of Series X Preferred  Units,  that the Partnership is or
likely is, or upon the  occurrence of a defined  event in the  immediate  future
will be or likely  will be a PTP,  or (z) the  holders of the Series X Preferred
Units hold or will hold 20% or more of the profits and capital  interests of the
Partnership,  provided  further  that,  in the case of clause (z),  the Series X
Preferred Units will be exchangeable  only to the extent necessary to reduce the
holdings of the holders of the Series X Preferred  Units to less than 20% of the
capital and profits interests of the Partnership.

        In  addition to and not in  limitation  of the  foregoing,  the Series X
Preferred Units may be exchanged for Series X Preferred Stock, in whole (and not
in part),  at the option of the  holders of a majority of the Series X Preferred
Units (acting as a whole) prior to the tenth (10th)  anniversary of the issuance
date and  after  the  third  anniversary  thereof  if such  holder  of  Series X
Preferred Units shall deliver to the General Partner either (i) a private letter
ruling  addressed to such holder of Series X Preferred  Units or (ii) an opinion
of independent counsel reasonably acceptable to the General Partner based on the
enactment of temporary or final  Treasury  Regulations  or the  publication of a
Revenue  Ruling in either  case to the effect  that an  exchange of the Series X
Preferred  Units at such  earlier  time would not cause the  Series X  Preferred
Units to be  considered  "stock and  securities"  within the  meaning of section
351(e) of the  Internal  Revenue  Code of 1986,  as  amended  (the  "Code")  for
purposes of determining  whether the holder of such Series X Preferred  Units is
an  "investment  company"  under  section  721(b) of the Code if an  exchange is
permitted at such earlier date.

        In  addition to and not in  limitation  of the  foregoing,  the Series X
Preferred  Units may be  exchanged  in whole  (and not in part)  (regardless  of
whether  held by Salomon  Smith  Barney  Tax  Advantaged  Exchange  Fund II, LLC
("Subscriber")  at the  option of the  holders  of a  majority  of the  Series X
Preferred  Units  (acting as a whole) for Series X Preferred  Stock (but only if
the  exchange  in whole  may be  accomplished  consistently  with the  ownership
limitations  set forth  under  the  Article  IV of the  Charter  of the  General
Partner,  taking  into  account  exceptions  thereto)  if at any  time  (i)  the
Partnership or the General  Partner breach any of the covenants set forth in the
Tax  Representations  Certificate  delivered  in  connection  with  the  Private
Placement Purchase  Agreement,  dated as of September 7, 1999, among Subscriber,
the  Partnership  and the  General  Partner,  (ii)  the  Partnership  reasonably
determines  that the  assets and income of the  Partnership  for a taxable  year
after 1999 would not satisfy  the income and assets  tests of Section 856 of the
Code for such  taxable  year if the  Partnership  were a real estate  investment
trust within the meaning of the Code, (iii) under the circumstances described in
the  penultimate  sentence  of  Section  6(b),  or (iv) any  holder  of Series X
Preferred  Units shall deliver to the  Partnership and the Company an opinion of
independent  counsel  reasonably  acceptable  to the Company to the effect that,
based on the assets and income of the Partnership for a taxable year after 1999,
the Partnership  would not satisfy the income and assets tests of Section 856 of
the Code for such taxable year if the Partnership were a real estate  investment
trust  within the meaning of the Code,  and that in the case of each of (ii) and
(iv),  such failure would create a meaningful risk that a holder of the Series X
Preferred Units would fail to maintain qualification as a real estate investment
trust.

        (ii)  Notwithstanding  anything  to the  contrary  set forth in  Section
8(a)(i),  if an Exchange Notice (as  hereinafter  defined) has been delivered to
the General  Partner,  then the  General  Partner  may, at its option,  elect to
redeem  or cause  the  Partnership  to  redeem  all (but not a  portion)  of the
outstanding  Series  X  Preferred  Units  for  cash in an  amount  equal  to the
Liquidation  Preference  per Series X Preferred  Unit.  The General  Partner may
exercise its option to redeem the Series X Preferred  Units for cash pursuant to
this  Section  8(a)(ii)  by giving  each  holder of record of Series X Preferred
Units notice of its election to redeem for cash,  within five (5) Business  Days
after  receipt of the  Exchange  Notice,  by (m) fax, and (n)  registered  mail,
postage  paid at the  address of each  holder as it may appear on the records of
the Partnership  stating (A) the redemption  date,  which shall be no later than
sixty (60) days following the receipt of the Exchange Notice, (B) the redemption
price,  (C) the place or places  where the  Series X  Preferred  Units are to be
surrendered for payment of the redemption  price, (D) that  distributions on the
Series X Preferred Units will cease to accrue on such redemption  date; (E) that
payment of the redemption price will be made upon  presentation and surrender of
the Series X Preferred Units and (F) the aggregate  number of Series X Preferred
Units to be redeemed.

        (iii) If an  exchange of Series X  Preferred  Units  pursuant to Section
8(a)(i)  would  violate the  provisions  on ownership  limitation of the General
Partner  set forth in Article  IV of the  Charter of the  General  Partner  with
respect to the Series X Preferred  Stock the General  Partner shall give written
notice thereof to each holder of record of Series X Preferred Units, within five
(5) Business Days following  receipt of the Exchange Notice, by (m) fax, and (n)
registered mail,  postage prepaid,  at the address of each such holder set forth
in the  records  of the  Partnership.  In such  event,  each  holder of Series X
Preferred  Units shall be entitled to exchange,  pursuant to the  provisions  of
Section  8(b) a number of Series X Preferred  Units which would  comply with the
provisions on the ownership  limitation of the General Partner set forth in such
Article IV of the  Charter of the  General  Partner  and any Series X  Preferred
Units not so exchanged (the "Excess Units") shall be redeemed by the Partnership
for cash in an amount equal to the Liquidation Preference. The written notice of
the  General  Partner  shall  state (A) the number of Excess  Units held by such
holder, (B) the redemption price of the Excess Units, (C) the date on which such
Excess  Units  shall be  redeemed,  which date shall be no later than sixty (60)
days following the receipt of the Exchange Notice, (D) the place or places where
such Excess Units are to be surrendered for payment of the Redemption Price, (E)
that  distributions  on the Excess Units will cease to accrue on such redemption
date,  and  (F)  that  payment  of  the  redemption  price  will  be  made  upon
presentation  and surrender of such Excess Units. If an exchange would result in
Excess Units, as a condition to such exchange,  each holder of such units agrees
to provide  representations  and covenants  reasonably  requested by the General
Partner  relating to (1) the widely held nature of the interests in such holder,
sufficient to assure the General Partner that the holder's ownership of stock of
the General  Partner  (without  regard to the limits  described  above) will not
cause any Person (as such term is defined in the  Articles of  Incorporation  of
the General Partner) to own stock of the General Partner in an amount that would
cause such Person not to comply with the provisions of the ownership  limitation
of the  General  Partner  set  forth  in  such  Article  IV of the  Articles  of
Incorporation of the General  Partner;  and (2) to the extent such holder can so
represent  and  covenant  without  obtaining  information  from its owners,  the
holder's ownership of tenants of the Partnership and its affiliates.

        Notwithstanding  any  provision of this  Agreement to the  contrary,  no
Series X Limited  Partner  shall be  entitled  to effect an exchange of Series X
Preferred  Units for Series X Preferred  Stock to the extent that  ownership  or
right to acquire  such shares would cause the Partner or any other Person or, in
the opinion of counsel selected by the General Partner, may cause the Partner or
any other Person to violate the restrictions on ownership and transfer of Series
X Preferred Stock set forth in the Articles of Incorporation.  To the extent any
such  attempted  exchange for Series X Preferred  Stock would be in violation of
the  previous  sentence,  it shall be void ab initio  and such  Series X Limited
Partner  shall not  acquire  any  rights or  economic  interest  in the Series X
Preferred Stock otherwise issuable upon such exchange.

        (iv) The  redemption  of Series X Preferred  Units  described in Section
8(a)(ii)  and (iii) shall be subject to the  provisions  of Section  5(b)(i) and
Section 5(b)(ii);  provided,  however,  that the term "redemption price" in such
Section shall be read to mean the Liquidation  Preference per Series X Preferred
Unit being redeemed.

        (b) Procedure for Exchange. (i) Any exchange shall be exercised pursuant
to a notice of exchange (the "Exchange Notice") delivered to the General Partner
by the holder  who is  exercising  such  exchange  right,  by (a) fax and (b) by
certified mail postage prepaid.  The exchange of Series X Preferred Units may be
effected  after the fifth (5th)  Business Day  following  receipt by the General
Partner of the Exchange Notice by delivering  certificates if any,  representing
such Series X Preferred  Units to be exchanged  together  with,  if  applicable,
written  notice of exchange and a proper  assignment  of such Series X Preferred
Units  to the  office  of the  General  Partner  maintained  for  such  purpose.
Currently,  such  office is c/o PS Business  Parks,  Inc.,  701 Western  Avenue,
Glendale,  California 91201, Attention:  Jack E. Corrigan. Each exchange will be
deemed to have been effected  immediately  prior to the close of business on the
date on which such Series X Preferred  Units to be exchanged  (together with all
required  documentation)  shall have been surrendered and notice shall have been
received by the General  Partner as aforesaid and the Exchange  Price shall have
been paid. Any Series X Preferred  Stock issued pursuant to this Section 8 shall
be delivered as shares which are duly authorized, validly issued, fully paid and
nonassessable, free of pledge, lien, encumbrance or restriction other than those
provided in the Charter,  the Bylaws of the General Partner,  the Securities Act
of 1933, as amended and relevant state securities or blue sky laws.

        (ii) In the event of an exchange of Series X Preferred  Units for shares
of Series X Preferred  Stock, an amount equal to the accrued and unpaid Priority
Return,  whether  or not  declared,  to the  date of  exchange  on any  Series X
Preferred  Units  tendered  for  exchange  shall (a) accrue on the shares of the
Series X Preferred Stock into which such Series X Preferred Units are exchanged,
and (b) continue to accrue on such Series X Preferred Units,  which shall remain
outstanding  following such exchange,  with the General Partner as the holder of
such Series X Preferred  Units.  Notwithstanding  anything to the  contrary  set
forth herein,  in no event shall a holder of a Series X Preferred  Unit that was
validly  exchanged into Series X Preferred Stock pursuant to this section (other
than the General  Partner now holding such Series X Preferred  Unit),  receive a
distribution from the Partnership,  if such holder, after exchange,  is entitled
to receive a distribution  from the General Partner with respect to the share of
Series X Preferred Stock for which such Series X Preferred Unit was exchanged or
redeemed.

        (iii) Fractional shares of Series X Preferred Stock are not to be issued
upon  exchange  but,  in lieu  thereof,  the  General  Partner  will  pay a cash
adjustment  based upon the fair market value of the Series X Preferred  Stock on
the day prior to the exchange  date as  determined in good faith by the Board of
Directors of the General Partner.

        (c) Adjustment of Exchange  Price.  (i) The Exchange Price is subject to
adjustment upon certain events,  including,  (a) subdivisions,  combinations and
reclassification  of the Series X Preferred Stock, and (b)  distributions to all
holders of Series X Preferred  Stock of evidences of indebtedness of the General
Partner  or  assets   (including   securities,   but  excluding   dividends  and
distributions  paid in cash out of  equity  applicable  to  Series  X  Preferred
Stock).

        (ii) In case the  General  Partner  shall be a party to any  transaction
(including,  without  limitation,  a  merger,  consolidation,   statutory  share
exchange,  tender offer for all or  substantially  all of the General  Partner's
capital  stock  or sale of all or  substantially  all of the  General  Partner's
assets),  in each case as a result of which the Series X Preferred Stock will be
converted into the right to receive shares of capital stock, other securities or
other  property  (including  cash or any  combination  thereof),  each  Series X
Preferred  Unit will  thereafter  be  exchangeable  into the kind and  amount of
shares of capital stock and other securities and property receivable  (including
cash or any combination  thereof) upon the consummation of such transaction by a
holder of that number of shares of Series X Preferred Stock or fraction  thereof
into which one Series X Preferred  Unit was  exchangeable  immediately  prior to
such  transaction.  The  General  Partner  may not  become  a party  to any such
transaction  unless the terms thereof are consistent with the foregoing.  In the
event of a conflict  between the  provisions  of this  Section  8(c)(ii) and any
provision of the Partnership Agreement,  the provisions of this Section 8(c)(ii)
shall control.

        Section 9. No Conversion  Rights.  Except as set forth in Section 8, the
holders of the  Series X  Preferred  Units  shall not have any rights to convert
such units into  shares of any other  class or series of stock or into any other
securities of, or interest in, the Partnership.

        Section 10. No Sinking  Fund. No sinking fund shall be  established  for
the retirement or redemption of Series X Preferred Units.

        Section 11. Exhibit A to Partnership Agreement. In order to duly reflect
the  issuance  of  the  Series  X  Preferred  Units  provided  for  herein,  the
Partnership Agreement is hereby further amended pursuant to Section 12.3 thereof
by deleting Exhibit A thereto and replacing Exhibit A attached hereto therefor.

        Section 12. Inconsistent  Provisions.  Nothing to the contrary contained
in the  Partnership  Agreement  shall limit any of the rights or obligations set
forth in this Amendment.

        IN WITNESS WHEREOF this Amendment has been executed as of the date first
above written.

                                        PS BUSINESS PARKS, INC.

                                        By: /s/ Jack Corrigan
                                            ----------------------
                                            Jack Corrigan
                                            Vice President and
                                            Chief Financial Officer