PS BUSINESS PARKS, L.P.

                        AMENDMENT TO AGREEMENT OF LIMITED
                             PARTNERSHIP RELATING TO
                      87/8% SERIES Y 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 12th day of July,  2000 (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 Y Preferred
Units,  $25.00 per Series Y 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 Y 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 Y
Preferred  Units (as  hereinafter  defined)  with respect to  distributions  and
rights upon voluntary or involuntary  liquidation,  winding-up or dissolution of
the Partnership,  including the 9 1/4% Series A Cumulative  Redeemable Preferred
Units,  the 8 7/8% Series B Cumulative  Redeemable  Preferred  Units, the 8 1/4%
Series  C  Cumulative  Redeemable  Preferred  Units  and  the  8 7/8%  Series  X
Cumulative Redeemable Preferred Units.  Notwithstanding the differing allocation
rights  set  forth  in  Section  4 below  that  apply to the  Series  A, B and C
Preferred  Units  (as  compared  to the  Series X and Y  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 and Y  Preferred
Units.

     (c)  "Priority  Return"  means an  amount  equal to 87/8%  per annum of the
Liquidation  Preference per Series Y Preferred  Unit,  commencing on the date of
issuance of such Series Y 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 Y 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 "87/8% Series Y Cumulative  Redeemable  Preferred  Units" (the
"Series Y  Preferred  Units")  is  hereby  established.  The  number of Series Y
Preferred Units shall be 480,000.  The Holders of Series Y 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 Y
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 Y 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,
2000, and (ii) in the event of a redemption of Series Y Preferred  Units (each a
"Series Y  Preferred  Unit  Distribution  Payment  Date").  If any date on which
distributions  are to be made on the Series Y 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 Y
Preferred  Units will be made to the holders of record of the Series Y 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 Y Preferred Unit Distribution Payment
Date (the "Series Y Preferred Unit Partnership Record Date").

     (b) Prohibition on  Distribution.  No  distributions  on Series Y 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 Y 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 Y Preferred Units will accumulate
as of the Series Y 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 Y Preferred  Unit  Distribution  Payment Date to holders of record of the
Series Y  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  Y  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  Y   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 Y Preferred  Units, any Parity Preferred Units
     or any Junior Units, unless, in each case, all distributions accumulated on
     all Series Y  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  Y  Preferred  Units  or (z) the
     redemption of  Partnership  Interests  corresponding  to Series Y 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 Y Preferred  Units, all  distributions  authorized
     and  declared on the Series Y 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 Y 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  Y  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 Y  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 and Y 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, X and Y 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, X and Y  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, X or Y  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 and Y  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 Y 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 Y 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   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 Y 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 Y Preferred
Units.  The Series Y 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 Y 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 Y 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 Y 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 Y 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 Y Preferred Units shall be delivered for payment of the
Redemption  Price, (q) that  distributions on the Series Y 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 Y  Preferred  Units as set forth in Section  5(b)(ii)
below.

     (ii) If the Partnership gives a notice of redemption in respect of Series Y
     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 Y 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 Y  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 Y 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 Y Preferred  Units  called for  redemption,  unless the  Partnership
     defaults in the payment thereof. If any date fixed for redemption of Series
     Y 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 Y
     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 Y 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 Y Preferred
Unit shall be treated as one Partnership Unit.)

     (b) Certain Voting Rights.  So long as any Series Y Preferred  Units remain
outstanding,  the Partnership  shall not,  without the  affirmative  vote of the
holders of at least a majority of the Series Y 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 Y
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 Y Preferred  Units and any other  series
that rank in parity  with the  Series Y  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 Y 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 Y 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 Y  Preferred  Units for
other interests in such entity having substantially the same terms and rights as
the Series Y 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 Y 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 Y 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 Y
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 Y  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 Y Preferred Units having  substantially  the same terms
and  rights  as  the  Series  Y  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 Y 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 Y Preferred Units shall be the acceleration of the exchange date relating
to the Series Y 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 Y 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 Y
Preferred Units may be transferred to a maximum of five (5) persons.  At no time
shall the number of holders of the Series Y Preferred Units exceed five.

     (b) Notwithstanding anything to the contrary in Section 7(a), if any holder
of Series Y 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 Y 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 Y 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  87/8%  Series  Y  Cumulative
Redeemable  Preferred  Stock of the  General  Partner  (the  "Series Y Preferred
Stock") at an  exchange  rate of one share of Series Y  Preferred  Stock for one
Series Y Preferred Unit, subject to adjustment as described below (the "Series Y
Exchange  Price");  provided  that the  Series Y  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 Y  Preferred  Units  (acting as a whole) for
Series Y Preferred  Stock if (x) at any time full  distributions  shall not have
been timely made on any Series Y  Preferred  Unit with  respect to six (6) prior
quarterly distribution periods, whether or not consecutive;  provided,  however,
that a distribution  in respect of Series Y Preferred  Units shall be considered
timely made if made within two (2) Business Days after the  applicable  Series Y
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 Y  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 Y 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 Y 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 Y
Preferred Units will be exchangeable  only to the extent necessary to reduce the
holdings of the holders of the Series Y 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  Y
Preferred Units may be exchanged for Series Y Preferred Stock, in whole (and not
in part),  at the option of the  holders of a majority of the Series Y 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 Y
Preferred Units shall deliver to the General Partner either (i) a private letter
ruling  addressed to such holder of Series Y 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 Y
Preferred  Units at such  earlier  time would not cause the  Series Y  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 Y 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  Y
Preferred  Units may be  exchanged  in whole  (and not in part)  (regardless  of
whether  held by Salomon  Smith  Barney Tax  Advantaged  Exchange  Fund III, LLC
("Subscriber")  at the  option of the  holders  of a  majority  of the  Series Y
Preferred  Units  (acting as a whole) for Series Y 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 July 12th, 2000, 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 2000
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 Y 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  2000,  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 Y
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 Y
Preferred  Units for cash in an amount equal to the  Liquidation  Preference per
Series Y Preferred  Unit. The General  Partner may exercise its option to redeem
the Series Y  Preferred  Units for cash  pursuant  to this  Section  8(a)(ii) by
giving each holder of record of Series Y 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 Y Preferred  Units are to be surrendered  for payment of
the redemption  price,  (D) that  distributions  on the Series Y 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 Y Preferred
Units and (F) the aggregate number of Series Y Preferred Units to be redeemed.


     (iii) If an  exchange  of Series Y  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 Y Preferred  Stock the General  Partner shall give written
notice thereof to each holder of record of Series Y 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 Y
Preferred  Units shall be entitled to exchange,  pursuant to the  provisions  of
Section  8(b) a number of Series Y 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 Y  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  provision of this  Agreement to the contrary,  no Series Y
Limited  Partner  shall be  entitled to effect an exchange of Series Y Preferred
Units for Series Y  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 Y
Preferred  Stock set forth in the Articles of  Incorporation.  To the extent any
such  attempted  exchange for Series Y Preferred  Stock would be in violation of
the  previous  sentence,  it shall be void ab initio  and such  Series Y Limited
Partner  shall not  acquire  any  rights or  economic  interest  in the Series Y
Preferred Stock otherwise issuable upon such exchange.


     (iv) The  redemption  of Series Y  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 Y 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 Y 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 Y Preferred  Units to be exchanged  together  with,  if  applicable,
written  notice of exchange and a proper  assignment  of such Series Y 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 Y 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 Y 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 Y Preferred  Units for shares of
Series Y Preferred  Stock,  an amount  equal to the accrued and unpaid  Priority
Return,  whether  or not  declared,  to the  date of  exchange  on any  Series Y
Preferred  Units  tendered  for  exchange  shall (a) accrue on the shares of the
Series Y Preferred Stock into which such Series Y Preferred Units are exchanged,
and (b) continue to accrue on such Series Y Preferred Units,  which shall remain
outstanding  following such exchange,  with the General Partner as the holder of
such Series Y Preferred  Units.  Notwithstanding  anything to the  contrary  set
forth herein,  in no event shall a holder of a Series Y Preferred  Unit that was
validly  exchanged into Series Y Preferred Stock pursuant to this section (other
than the General  Partner now holding such Series Y 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 Y Preferred Stock for which such Series Y Preferred Unit was exchanged or
redeemed.

     (iii)  Fractional  shares of Series Y 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 Y 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 Y Preferred Stock, and (b)  distributions to all
holders of Series Y 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  Y  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 Y 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 Y
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 Y Preferred Stock or fraction  thereof
into which one Series Y 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 Y  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 Y Preferred Units.

     Section 11.  Exhibit A to Partnership  Agreement.  In order to duly reflect
the  issuance  of  the  Series  Y  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