Exhibit 31.1


                            CERTIFICATION PURSUANT TO
              RULE 13a-14(a) OF THE SECURITIES EXCHANGE ACT OF 1934

I, Carl E. Berg, certify that:

1.   I have reviewed this annual report on Form 10-K of Mission West Properties,
     Inc.;

2.   Based on my knowledge, this report does not contain any untrue statement of
     a material  fact or omit to state a  material  fact  necessary  to make the
     statements made, in light of the circumstances  under which such statements
     were made,  not  misleading  with  respect  to the  period  covered by this
     report;

3.   Based on my  knowledge,  the  financial  statements,  and  other  financial
     information  included  in  this  report,  fairly  present  in all  material
     respects the financial  condition,  results of operations and cash flows of
     the registrant as of, and for, the periods presented in this report;

4.   The  registrant's  other  certifying  officers  and I are  responsible  for
     establishing and maintaining disclosure controls and procedures (as defined
     in Exchange Act Rules  13a-15(e) and 15d-15(e))  and internal  control over
     financial  reporting  (as  defined  in  Exchange  Act Rules  13a-15(f)  and
     15d-15(f)) for the registrant and have:

     (a)  Designed  such  disclosure  controls  and  procedures,  or caused such
          disclosure   controls  and   procedures  to  be  designed   under  our
          supervision,  to ensure  that  material  information  relating  to the
          registrant,  including its consolidated subsidiaries, is made known to
          us by others within those entities,  particularly during the period in
          which this report is being prepared;
     (b)  Designed such internal  control over  financial  reporting,  or caused
          such internal  control over  financial  reporting to be designed under
          our  supervision,   to  provide  reasonable  assurance  regarding  the
          reliability  of financial  reporting and the  preparation of financial
          statements for external purposes in accordance with generally accepted
          accounting principles;
     (c)  Evaluated the  effectiveness of the registrant's  disclosure  controls
          and procedures and presented in this report our conclusions  about the
          effectiveness of the disclosure controls and procedures, as of the end
          of the period covered by this report based on such evaluation; and
     (d)  Disclosed  in this  report  any  change in the  registrant's  internal
          control over financial reporting that occurred during the registrant's
          most recent fiscal quarter (the registrant's  fourth fiscal quarter in
          the case of an annual  report)  that has  materially  affected,  or is
          reasonably  likely to materially  affect,  the  registrant's  internal
          control over financial reporting; and

5.   The registrant's other certifying  officers and I have disclosed,  based on
     our most recent evaluation of internal control over financial reporting, to
     the registrant's auditors and the audit committee of the registrant's board
     of directors (or persons performing the equivalent functions):

     (a)  All significant  deficiencies and material weaknesses in the design or
          operation  of internal  control  over  financial  reporting  which are
          reasonably  likely to  adversely  affect the  registrant's  ability to
          record, process, summarize and report financial information; and
     (b)  Any fraud, whether or not material,  that involves management or other
          employees who have a  significant  role in the  registrant's  internal
          control over financial reporting.


Date: March 14, 2006                       By: /s/ Carl E. Berg
                                              ----------------------------------
                                              Carl E. Berg
                                              Chairman & Chief Executive Officer

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