Exhibit 99.1350CERT

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

I, Robert C. Doll, Jr., Chief Executive Officer of S&P 500 Index Fund of Merrill
Lynch Index Funds, Inc. and Master S&P 500 Index Series of Quantitative Master
Series Trust (together, the "Fund"), certify that:

      1.    The N-CSR of the Fund (the "Report") fully complies with the
            requirements of Section 13(a) or 15(d) of the Securities Exchange
            Act of 1934; and

      2.    The information contained in the Report fairly presents, in all
            material respects, the financial condition and results of operations
            of the Fund.

      Dated: February 24, 2005


                                                   /s/ Robert C. Doll, Jr.
                                                   -----------------------
                                                   Robert C. Doll, Jr.,
                                                   Chief Executive Officer of
                                                   S&P 500 Index Fund of Merrill
                                                   Lynch Index Funds, Inc. and
                                                   Master S&P 500 Index Series
                                                   of Quantitative Master Series
                                                   Trust

A signed original of this written statement required by Section 906, or other
document authenticating, acknowledging, or otherwise adopting the signature that
appears in typed form within the electronic version of this written statement
required by Section 906, has been provided to S&P 500 Index Fund of Merrill
Lynch Index Funds, Inc. and Master S&P 500 Index Series of Quantitative Master
Series Trust and will be retained by S&P 500 Index Fund of Merrill Lynch Index
Funds, Inc. and Master S&P 500 Index Series of Quantitative Master Series Trust
and furnished to the Securities and Exchange Commission or its staff upon
request.



Exhibit 99.1350CERT

         Certification Pursuant to Section 906 of the Sarbanes Oxley Act

I, Donald C. Burke, Chief Financial Officer of S&P 500 Index Fund of Merrill
Lynch Index Funds, Inc. and Master S&P 500 Index Series of Quantitative Master
Series Trust (together, the "Fund"), certify that:

      1.    The N-CSR of the Fund (the "Report") fully complies with the
            requirements of Section 13(a) or 15(d) of the Securities Exchange
            Act of 1934; and

      2.    The information contained in the Report fairly presents, in all
            material respects, the financial condition and results of operations
            of the Fund.

      Dated: February 24, 2005


                                                   /s/ Donald C. Burke
                                                   -------------------
                                                   Donald C. Burke,
                                                   Chief Financial Officer of
                                                   S&P 500 Index Fund of Merrill
                                                   Lynch Index Funds, Inc. and
                                                   Master S&P 500 Index Series
                                                   of Quantitative Master Series
                                                   Trust

A signed original of this written statement required by Section 906, or other
document authenticating, acknowledging, or otherwise adopting the signature that
appears in typed form within the electronic version of this written statement
required by Section 906, has been provided to S&P 500 Index Fund of Merrill
Lynch Index Funds, Inc. and Master S&P 500 Index Series of Quantitative Master
Series Trust and will be retained by S&P 500 Index Fund of Merrill Lynch Index
Funds, Inc. and Master S&P 500 Index Series of Quantitative Master Series Trust
and furnished to the Securities and Exchange Commission or its staff upon
request.