STATEMENT OF ADDITIONAL INFORMATION BlackRock Index Funds, Inc. iShares Russell 2000 Small-Cap Index Fund iShares MSCI EAFE International Index Fund 100 Bellevue Parkway, Wilmington, Delaware • Phone No. (800) 441-7762 BlackRock Index Funds, Inc. (the “Corporation”) currently consists of two operating portfolios or series, iShares Russell 2000 Small-Cap Index Fund (“Russell 2000 Small-Cap Index Fund”) and iShares MSCI EAFE International Index Fund (“MSCI EAFE International Index Fund” and together with Russell 2000 Small-Cap Index Fund, the “Funds”). Russell 2000 Small-Cap Index Fund invests all of its assets in Master Small Cap Index Series of Quantitative Master Series LLC (the “Master LLC”) that has the same investment objective as Russell 2000 Small-Cap Index Fund. This Statement of Additional Information of the Funds is not a prospectus and should be read in conjunction with the prospectuses of the Funds, each dated April 30, 2021, as they may be amended or supplemented from time to time, (the “Prospectuses”), which have been filed with the Securities and Exchange Commission (the “Commission”) and can be obtained, without charge, by calling (800) 441-7762 or by writing to the Funds at the above address. The Prospectuses are incorporated by reference into this Statement of Additional Information, and Part I of this Statement of Additional Information and the portions of Part II of this Statement of Additional Information that relate to a Fund have been incorporated by reference into the Fund’s Prospectuses. The portions of Part II of this Statement of Additional Information that do not relate to a Fund do not form a part of that Fund’s Statement of Additional Information, have not been incorporated by reference into that Fund’s Prospectuses and should not be relied upon by investors in that Fund. The audited financial statements of the Funds and Master Small Cap Index Series are incorporated into this Statement of Additional Information by reference to the Funds’ Annual Report to Shareholders for the fiscal year ended December 31, 2020 (“Annual Report”). You may request a copy of an Annual Report at no charge by calling (800) 441-7762 between 8:00 a.m. and 6:00 p.m. Eastern time on any business day. References to the Investment Company Act of 1940, as amended (the “Investment Company Act”), or other applicable law, will include any rules promulgated thereunder and any guidance, interpretations or modifications by the Commission, Commission staff or other authority with appropriate jurisdiction, including court interpretations, and exemptive, no-action or other relief or permission from the Commission, Commission staff or other authority. iShares Russell 2000 iShares MSCI EAFE Small-Cap International Index Fund: Index Fund: Class Ticker Symbol Ticker Symbol Investor A Shares........................... MDSKX MDIIX Institutional Shares ........................ MASKX MAIIX Investor P Shares .......................... BDBPX BTMPX Class K Shares ............................. BDBKX BTMKX BlackRock Advisors, LLC — Manager BlackRock Investments, LLC — Distributor The date of this Statement of Additional Information is April 30, 2021. TABLE OF CONTENTS Page PART I InvestmentObjectivesandPolicies.................................................................. I-1 InvestmentRestrictions............................................................................ I-8 InformationonDirectorsandOfficers................................................................ I-9 Management, Advisory and Other Service Arrangements . ....................................... I-20 Information on Sales Charges and Distribution Related Expenses . ....................................... I-26 Computation of Offering Price Per Share . .......................................... I-27 Portfolio Transactions and Brokerage . .............................................................. I-27 Additional Information. ........................................................... I-30 Financial Statements .............................................................................. I-32 PART II Investment Risks and Considerations . ........................................................ II-1 Management and Other Service Arrangements . ........................................ II-72 Selective Disclosure of Portfolio Holdings. .......................................... II-75 Purchase of Shares................................................................................ II-87 Redemption of Shares . ........................................................................... II-104 Shareholder Services . ............................................................................ II-107 Pricing of Shares .................................................................................. II-111 Portfolio Transactions and Brokerage . .............................................................. II-113 Dividends and Taxes . ............................................................................. II-117 Performance Data ................................................................................. II-124 Proxy Voting Policies and Procedures. ........................................... II-126 General Information ............................................................................... II-126 Appendix A — Description of Bond Ratings. ........................................................ A-1 Appendix B — Proxy Voting Policies . ............................................................ B-1 PART I: INFORMATION ABOUT ISHARES RUSSELL 2000 SMALL-CAP INDEX FUND AND ISHARES MSCI EAFE INTERNATIONAL INDEX FUND Part I of this Statement of Additional Information (“SAI”) sets forth information about iShares Russell 2000 Small-Cap Index Fund (“Russell 2000 Small Cap Index Fund”) and iShares MSCI EAFE International Index Fund (“MSCI EAFE International Index Fund”) (each a “Fund” and together the “Funds”), each a series of BlackRock Index Funds, Inc. (the “Corporation”). It includes information about the Corporation’s Board of Directors (the “Board,” and the directors thereof, the “Directors”), the advisory and management services provided to and the management fees applicable to the Funds, performance data for the Funds and information about other fees applicable to and services provided to the Funds. This Part I of this SAI should be read in conjunction with the Funds’ Prospectuses and those portions of Part II of this SAI that pertain to the Funds. I. Investment Objectives and Policies The Corporation currently consists of two operating series, Russell 2000 Small-Cap Index Fund and MSCI EAFE International Index Fund. Each Fund’s investment objective is to provide investment results that, before expenses, replicate the total return (i.e., the combination of capital changes and income) of a specified securities index. Each Fund is classified as a diversified open-end management investment company under the Investment Company Act of 1940, as amended (the “Investment Company Act”). Russell 2000 Small-Cap Index Fund seeks to achieve its objective by investing all of its assets in Master Small Cap Index Series of Quantitative Master Series LLC (the “Master LLC”) that has the same investment objective as Russell 2000 Small Cap Index Fund. Russell 2000 Small-Cap Index Fund’s investment experience and results will correspond directly to the investment experience of Master Small Cap Index Series. Thus, all of Russell 2000 Small- Cap Index Fund’s investments are made at the level of Master Small Cap Index Series. For simplicity, however, with respect to investment objectives, policies and restrictions, this SAI, like the Prospectus, uses the terms “Fund,” as applicable, and “Russell 2000 Small-Cap Index Fund” to include Master Small Cap Index Series. Reference is made to the discussion under “How Each Fund Invests” in the Prospectus for information with respect to each Fund’s and Master Small Cap Index Series’ investment objectives and policies. There can be no assurance that the investment objectives of the Funds will be achieved. The Funds’ investment objectives are not fundamental policies and may be changed by the Board without shareholder approval. The Directors may also change the target index of a Fund if they believe that a different index would facilitate the management of the Fund in a manner which better enables the Fund to seek to replicate the total return of the market segment represented by the current index. The Funds that are subject to Rule 35d-1 under the Investment Company Act, will not change their investment policies required by that rule without giving shareholders 60 days’ prior written notice. Russell 2000 Small-Cap Index Fund The investment objective of the Russell 2000 Small-Cap Index Fund is to match the performance of the Russell 2000® Index (the “Russell 2000”) as closely as possible before the deduction of Fund expenses. There can be no assurance that the investment objective of the Fund will be achieved. The Fund seeks to achieve its investment objective by investing all of its assets in the Master Small Cap Index Series of the Master LLC, which has the same investment objective as the Fund. The following is a description of the investment policies of Russell 2000 Small-Cap Index Fund. In seeking to replicate the total return of the Russell 2000, BlackRock Advisors, LLC (“BlackRock” or the “Manager”) may not allocate the Russell 2000 Small-Cap Index Fund’s investments among all of the common stocks in the Russell 2000, or in the same weightings