EX-99.CERT

                      CERTIFICATION PURSUANT TO SECTION 302
                        OF THE SARBANES-OXLEY ACT OF 2002

- --------------------------------------------------------------------------------

I, Robert C. Doll, Jr., Chief Executive Officer of Merrill Lynch Funds for
Institutions Series and Master Institutional Money Market Trust, certify that:

     1.   I have reviewed this report on Form N-CSR of Merrill Lynch Funds for
          Institutions Series and Master Institutional Money Market Trust;

     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, changes in
          net assets, and cash flows (if the financial statements are required
          to include a statement of cash flows) of the registrants as of, and
          for, the periods presented in this report;

     4.   The registrants' other certifying officer(s) and I are responsible for
          establishing and maintaining disclosure controls and procedures (as
          defined in Rule 30a-3(c) under the Investment Company Act of 1940) and
          internal control over financial reporting (as defined in Rule 30a-3(d)
          under the Investment Company Act of 1940) for the registrants 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 registrants' disclosure
               controls and procedures and presented in this report our
               conclusions about the effectiveness of the disclosure controls
               and procedures, as of a date within 90 days prior to the filing
               date of this report based on such evaluation; and

          d)   disclosed in this report any change in the registrants' internal
               control over financial reporting that occurred during the second
               fiscal quarter of the period covered by this report that has
               materially affected, or is reasonably likely to materially
               affect, the registrants' internal control over financial
               reporting; and

     5.   The registrants' other certifying officer(s) and I have disclosed to
          the registrants' auditors and the audit committees of the registrants'
          boards 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 registrants'
               internal control over financial reporting.

Date: December 16, 2005

                                         /s/ Robert C. Doll, Jr.
                                         --------------------
                                         Robert C. Doll, Jr.,
                                         Chief Executive Officer of
                                         Merrill Lynch Funds for Institutions
                                         Series and Master Institutional
                                         Money Market Trust




EX-99.CERT

                      CERTIFICATION PURSUANT TO SECTION 302
                        OF THE SARBANES-OXLEY ACT OF 2002

- --------------------------------------------------------------------------------

I, William M. Breen, Chief Financial Officer of Merrill Lynch Funds for
Institutions Series and Master Institutional Money Market Trust, certify that:

     1.   I have reviewed this report on Form N-CSR of Merrill Lynch Funds for
          Institutions Series and Master Institutional Money Market Trust;

     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, changes in
          net assets, and cash flows (if the financial statements are required
          to include a statement of cash flows) of the registrants as of, and
          for, the periods presented in this report;

     4.   The registrants' other certifying officer(s) and I are responsible for
          establishing and maintaining disclosure controls and procedures (as
          defined in Rule 30a-3(c) under the Investment Company Act of 1940) and
          internal control over financial reporting (as defined in Rule 30a-3(d)
          under the Investment Company Act of 1940) for the registrants 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 registrants' disclosure
               controls and procedures and presented in this report our
               conclusions about the effectiveness of the disclosure controls
               and procedures, as of a date within 90 days prior to the filing
               date of this report based on such evaluation; and

          d)   disclosed in this report any change in the registrants' internal
               control over financial reporting that occurred during the second
               fiscal quarter of the period covered by this report that has
               materially affected, or is reasonably likely to materially
               affect, the registrants' internal control over financial
               reporting; and

     5.   The registrants' other certifying officer(s) and I have disclosed to
          the registrants' auditors and the audit committees of the registrants'
          boards 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 registrants'
               internal control over financial reporting.

Date: December 16, 2005

                                         /s/ William M. Breen
                                         -------------------
                                         William M. Breen,
                                         Chief Financial Officer of
                                         Merrill Lynch Funds for Institutions
                                         Series and Master Institutional
                                         Money Market Trust