Exhibit 10.23

                            PS BUSINESS PARKS, L.P.
                        AMENDMENT TO AGREEMENT OF LIMITED
                             PARTNERSHIP RELATING TO
                      8 3/4% SERIES C 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 3rd 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 by that  certain  Amendment to
Agreement  of  Limited  Partnership  Relating  to 8  3/4%  Series  B  Cumulative
Redeemable  Preferred  Units,  dated as of April 23,  1999 and an  Amendment  to
Agreement  of  Limited  Partnership  Relating  to 9  1/4%  Series  A  Cumulative
Redeemable  Preferred  Units,  dated as of April 30,  1999 and as may be further
amended by an  Amendment  Relating to Series X Cumulative  Redeemable  Preferred
Units which is expected to be generally in a form similar to the Amendment dated
as of  April  23,  1999 and may be  entered  into at or  about  the time  hereof
(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,  Operating  Partnership  and  Company  intend to issue  certain
Series X Preferred Units and to authorize issuance of the accompanying  Series X
Preferred  Shares,  respectively,  either soon before or after the date  hereof,
which  Series X  Preferred  Units and  Series X  Preferred  Shares,  when and if
issued,  shall rank in parity with or junior to the Series C Preferred Units and
Series C  Preferred  Shares  with  respect  to  distributions  and  rights  upon
voluntary  or   involuntary   liquidation   winding-up  or  dissolution  of  the
Partnership;

        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  C
Preferred  Units,  $25.00 per Series C  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 C 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 on a parity with the Series
C 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  and the 8 3/4%  Series  B  Cumulative  Redeemable  Preferred  Units.  For
purposes of this  Amendment,  the Series X Preferred  Units shall be  considered
Parity Preferred Units and shall not be considered  "Junior Units" as defined in
Section 3(d)(i) below,  notwithstanding the differing allocation  provisions set
forth in Section 4 herein.

        (c)  "Priority  Return" means an amount equal to 8 3/4% per annum of the
Liquidation  Preference per Series C Preferred  Unit,  commencing on the date of
issuance of such Series C Preferred  Unit,  determined on the basis of a 360-day
year of twelve 30-day  months,  and for any period shorter than a full quarterly
period for which  distributions  are computed,  the amount of the  distributions
payable will be based on the ratio of the actual  number of days elapsed in such
period to ninety (90) days,  cumulative  to the extent not  distributed  for any
given distribution period pursuant to Section 3, hereof,  commencing on the date
of the issuance of such Series C Preferred Unit.

        (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 3/4% Series C Cumulative  Redeemable  Preferred Units" (the
"Series C  Preferred  Units")  is  hereby  established.  The  number of Series C
Preferred  Units shall be  3,200,000.  The  Holders of Series C 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 C
Preferred  Units shall be entitled to receive,  when,  as and if declared by the
Partnership  acting  through the General  Partner,  the  Priority  Return.  Such
Priority  Return shall be  cumulative,  shall  accrue from the original  date of
issuance of the Series C Preferred Units and, notwithstanding Section 5.1 of the
Partnership Agreement, will be payable (i) quarterly in arrears on March 1, June
1,  September 1 and December 1 of each year  commencing on December 1, 1999, and
(ii) in the event of (A) a  redemption  of Series C Preferred  Units,  or (B) an
exchange of Series C  Preferred  Units into  Series C  Preferred  Stock,  on the
redemption date or the exchange date, as applicable  (each a "Series C Preferred
Unit Distribution Payment Date"). The amount of the distribution payable for any
period will be computed on the basis of a 360-day year of twelve  30-day  months
and for any period shorter than a full quarterly period for which  distributions
are computed,  the amount of the distribution  payable will be computed based on
the ratio of the actual  number of days  elapsed in such  period to ninety  (90)
days.  If any  date  on  which  distributions  are to be made  on the  Series  C
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 C  Preferred  Units will be made to the
holders of record of the Series C 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 C Preferred Unit Distribution  Payment Date (the "Series C Preferred Unit
Partnership Record Date").

        (b) Prohibition on Distribution.  No distributions on Series C 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 C Preferred
Units  will  accrue,  whether  or not  declared,  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 C
Preferred Units will  accumulate as of the Series C 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 C Preferred Unit Distribution Payment Date
to holders of record of the Series C 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  C  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 C 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 C Preferred
Units, any Parity Preferred Units or any Junior Units, unless, in each case, all
distributions  accumulated  on all Series C 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,  or (y) the  conversion  of Junior Units or Parity  Preferred  Units into
Partnership  Interests  ranking  junior to the  Series C  Preferred  Units as to
distributions and rights upon involuntary or voluntary liquidation,  dissolution
or winding up of the Partnership or (z) the redemption of Partnership  Interests
corresponding  to Series C Preferred  Stock,  Parity  Preferred  Stock or Junior
Stock to be  purchased  by the  General  Partner  pursuant  to the  Articles  of
Incorporation  of the General  Partner  with  respect to the  General  Partner's
common stock and  comparable  provisions in the Articles of  Incorporation  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 C Preferred  Units,  all  distributions  authorized and
declared  on the  Series  C  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 C 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 C 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 C 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 herein,  the Series C 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 C  Preferred
Units,  in whole (and not in part),  at any time, upon not less than 30 nor more
than 60 days written notice,  at a redemption  price,  payable in cash, equal to
the  Liquidation  Preference (the "Series C 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 C 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 C Preferred
Units.

        (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 30 nor more than 60 days prior to the redemption  date,  addressed
to the  respective  holders of record of the Series C  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 C 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 C 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 C Preferred Units shall be
delivered for payment of the Redemption  Price,  (q) that  distributions  on the
Series C  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 C Preferred  Units as
set forth in Section 5(b)(ii) below.

        (ii) If the  Partnership  gives a notice of  redemption  in  respect  of
Series C 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 C  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 C 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 C  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 C Preferred  Units called for  redemption,  unless the
Partnership defaults in the payment thereof. If any date fixed for redemption of
Series C 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 C 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 C 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 C Preferred
Unit  shall  be  treated  as one  Partnership  Unit.)  If and for so long as the
General  Partner holds any Series C Preferred  Units,  the General Partner shall
not have any voting  rights with  respect to such  Series C Preferred  Units and
such Series C Preferred  Units shall not be counted in determining the number of
such units  outstanding  for the purpose of  determining  whether the holders of
such units have granted any approval called for hereunder.

        (b)  Certain  Voting  Rights.  So long as any Series C  Preferred  Units
remain  outstanding,  the Partnership shall not, without the affirmative vote of
the holders of at least a majority of the Series C 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 C
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;

        (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 such
series of 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
materially  and  adversely  affect  the  powers,  special  rights,  preferences,
privileges  or  voting  power of the  Series C  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 C  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 C  Preferred  Units for
other interests in such entity having substantially the same terms and rights as
the Series C 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 C 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 C 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 C
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  materially  and  adversely  affect such  rights,  preferences,
privileges or voting powers.

        In addition to the foregoing,  the  Partnership  will not (x) enter into
any contract,  mortgage, loan or other agreement that prohibits or restricts, or
has the effect of  prohibiting  or  restricting,  the ability of a holder of the
Preferred  Units to  exercise  its rights set forth  herein to effect in full an
exchange or  redemption  pursuant  to Section 8, below,  except with the written
consent of such holder; or (y) amend,  alter, or repeal or waive Sections 7.6 or
11.3(f) of the Partnership  Agreement without the affirmative vote of at least a
majority of the Series C Preferred Units outstanding at the time.

        Section 7.  Transfer  Restrictions.  The  holders of Series C  Preferred
Units shall be subject to all of the provisions of Section 11 of the Partnership
Agreement  except  Section  11.3(b),  as modified by this Section 7. The General
Partner  shall not  unreasonably  withhold  consent  to a transfer  required  by
Section 11.3 (a). Subject to the consent of the General Partner, which shall not
be  unreasonably  withheld  or  delayed,  the  Series C  Preferred  Units may be
transferred  to a maximum  of five (5)  persons.  At no time shall the number of
holders of the Series C Preferred Units exceed five.

        Section  8.  Exchange  Rights.  (a)  Right  to  Exchange.  (i)  Series C
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  3/4%  Series  C  Cumulative
Redeemable  Preferred  Stock of the  General  Partner  (the  "Series C Preferred
Stock") at an  exchange  rate of one share of Series C  Preferred  Stock for one
Series C Preferred Unit, subject to adjustment as described below (the "Series C
Exchange  Price");  provided  that the  Series C  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 C  Preferred  Units  (acting as a whole) for
Series C Preferred  Stock if (x) at any time full  distributions  shall not have
been timely made on any Series C  Preferred  Unit with  respect to six (6) prior
quarterly distribution periods, whether or not consecutive;  provided,  however,
that a distribution  in respect of Series C Preferred  Units shall be considered
timely made if made within two (2) Business Days after the  applicable  Series C
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 or (y) upon  receipt  by a holder or
holders of Series C Preferred  Units of (1) notice from the General Partner that
the General  Partner or a Subsidiary of the General  Partner has become aware of
facts that will or likely will cause the Partnership to become 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 C
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. In
addition to and not in limitation of the foregoing, the Series C Preferred Units
may be exchanged for Series C Preferred  Stock,  in whole (and not in part),  at
the option of the holders of a majority of the Series C 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 C Preferred Units shall
deliver to the General  Partner either (i) a private letter ruling  addressed to
such  holder of  Series C  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  since  the date of  Closing  or the
publication  of a Revenue Ruling since the date of Closing in either case to the
effect that an exchange of the Series C  Preferred  Units at such  earlier  time
would  not  cause the  Series C  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 C 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 C
Preferred  Units may be  exchanged  in whole  (and not in part)  (regardless  of
whether held by  Contributor)  at the option of the holders of a majority of the
Series C Preferred  Units (acting as a whole) for Series C 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 either
(x) the  General  Partner  has  provided  notice to the  holders of the Series C
Preferred  Units pursuant to Section 4(e) of the  Contribution  Agreement by and
among the  Contributor,  LLC, the General  Partner and the Partnership or (y)(i)
such holders  conclude  based on results or projected  results that there exists
(in the reasonable  judgment of such holders) an imminent and  substantial  risk
that the Series C Preferred Units represent or will represent more than 18.0% of
the total profits of or capital interests in the Partnership for a taxable year,
(ii) such  holders  deliver to the  General  Partner  an  opinion of  nationally
recognized independent counsel, reasonably acceptable to the General Partner, to
the effect that there is a substantial risk that its interest in the Partnership
does not or will not satisfy  the 18.0%  limit,  and (iii) the  General  Partner
agrees  with  the  conclusions  referred  to in  clauses  (i)  and  (ii) of this
sentence, such agreement not to be unreasonably withheld.

        (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  C  Preferred  Units  for  cash in an  amount  equal  to the
Liquidation  Preference  per Series C Preferred  Unit.  The General  Partner may
exercise its option to redeem the Series C Preferred  Units for cash pursuant to
this  Section  8(a)(ii)  by giving  each  holder of record of Series C 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 C  Preferred  Units are to be
surrendered for payment of the redemption  price, (D) that  distributions on the
Series C 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 C Preferred Units and (F) the aggregate  number of Series C Preferred
Units to be redeemed.

        (iii) If an  exchange of Series C  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 C Preferred  Stock the General  Partner shall give written
notice thereof to each holder of record of Series C 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 C
Preferred  Units shall be entitled to exchange,  pursuant to the  provisions  of
Section  8(b) a number of Series C 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 C  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 Charter 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.

        To the extent the General  Partner would not be able to pay the cash set
forth above in exchange for the Excess Units, and to the extent  consistent with
the Articles of Incorporation,  the General Partner agrees that it will grant to
the  holders  of the  Series C  Preferred  Units  exceptions  to the  Beneficial
Ownership  Limit and  Constructive  Ownership  Limit  set forth in the  Series C

Certificate of Determination sufficient to allow such holders to exchange all of
their  Series C Preferred  Units for Series C  Preferred  Stock,  provided  such
holders furnish to the General Partner representations acceptable to the General
Partner in its sole and absolute  discretion  which  assure the General  Partner
that such exceptions  will not jeopardize the General  Partner's tax status as a
REIT for purposes of federal and applicable state law.

        Notwithstanding  any  provision  of the  Agreement to the  contrary,  no
Series C Limited  Partner  shall be  entitled  to effect an exchange of Series C
Preferred  Units for Series C 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 C  Preferred  Stock set forth in the  Articles of  Incorporation.  To the
extent any such  attempted  exchange  for Series C  Preferred  Stock would be in
violation of the previous sentence, it shall be void ab initio and such Series C
Limited Partner shall not acquire any rights or economic  interest in the Series
C Preferred Stock otherwise issuable upon such exchange.

        (iv) The  redemption of Series C Preferred  Units  described in Sections
8(a) (ii) and (iii) shall be subject to the  provisions of Sections  5(b)(i) and
(ii); provided,  however, that the term "redemption price" in such Section shall
be read to mean the  Liquidation  Preference  per Series C 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 C 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 C Preferred  Units to be exchanged  together  with,  if  applicable,
written  notice of exchange and a proper  assignment  of such Series C 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 C 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 C 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 C Preferred  Units for shares
of Series C Preferred  Stock, an amount equal to the accrued and unpaid Priority
Return,  whether  or not  declared,  to the  date of  exchange  on any  Series C
Preferred  Units  tendered  for  exchange  shall (a) accrue on the shares of the
Series C Preferred Stock into which such Series C Preferred Units are exchanged,
and (b) continue to accrue on such Series C Preferred Units,  which shall remain
outstanding  following such exchange,  with the General Partner as the holder of
such Series C Preferred  Units.  Notwithstanding  anything to the  contrary  set
forth herein,  in no event shall a holder of a Series C Preferred  Unit that was
validly  exchanged into Series C Preferred Stock pursuant to this section (other
than the General  Partner now holding such Series C 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 C Preferred Stock for which such Series C Preferred Unit was exchanged or
redeemed.

        (iii) Fractional shares of Series C 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 C 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 C Preferred Stock, and (b)  distributions to all
holders of Series C 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  C  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 C 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 C
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 C Preferred Stock or fraction  thereof
into which one Series C 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
addition,  so  long  as a  Series  C  Limited  Partner  or any of its  permitted
successors or assigns,  hold any Series C Preferred  Units,  as the case may be,
the General Partner shall not, without the affirmative vote of the holders of at
least a majority of the Series C Preferred  Units  outstanding  at the time: (a)
designate or create,  or increase the  authorized or issued amount of, any class
or series of shares ranking senior to the Series C Preferred  Stock with respect
to the payment of  distributions  or rights  upon  liquidation,  dissolution  or
winding-up or reclassify any authorized  shares of the General  Partner into any
such  shares,  or  create,  authorize  or issue any  obligations  or  securities
convertible  into or  evidencing  the right to purchase any such shares;  or (b)
amend,  alter or repeal the  provisions  of the Charter or bylaws of the General
Partner,  whether by merger,  consolidation or otherwise,  that would materially
and adversely  affect the powers,  special  rights,  preferences,  privileges or
voting power of the Series C Preferred Stock or the holders  thereof;  provided,
however,, that any increase in the amount of authorized  Preferred Shares or the
creation or issuance of any other  series or class of Preferred  Shares,  or any
increase  in the amount of  authorized  shares of each class or series,  in each
case ranking  either (1) junior to the Series C Preferred  Stock with respect to
the payment of distributions  and the  distribution of assets upon  liquidation,
dissolution or winding-up,  or (2) on a parity with the Series C Preferred Stock
with respect to the payment of distributions and the distribution of assets upon
liquidation,  dissolution  or winding-up  shall not be deemed to materially  and
adversely affect such rights,  preferences,  privileges or voting powers. 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 C  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 C Preferred Units.

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