UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

Amendment No. 1

For the fiscal year ended December 31, 2011

For the transition period from              to             

Commission file numbers: 001-32947 (Registrant)

                                                     001-32948 (Co-Registrant)

iShares^® S&P GSCI™ Commodity-Indexed Trust

iShares^® S&P GSCI™ Commodity-Indexed Investing Pool LLC

(Rule 140 Co-Registrant)

(Exact name of Registrant as specified in its charter)

c/o BlackRock Asset Management International Inc.

400 Howard Street

San Francisco, California 94105

Attn: Product Management Team

iShares Product Research & Development

(Address of principal executive offices)

(415) 670-2000

(Registrant and Co-Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the Registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  x

Indicate by check mark if the Registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the Registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the Registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the Registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the Registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§229.405 of this chapter) is not contained herein, and will not be contained, to the best of Registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  x

Indicate by check mark whether the Registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the Registrant and Co-Registrant are shell companies (as defined in Rule 12b-2 of the Act).    Yes  ¨    No  x

As of June 30, 2011, the aggregate market value of the shares held by non-affiliates was approximately $1,524,717,000.

DOCUMENTS INCORPORATED BY REFERENCE:

None.

THE FINANCIAL STATEMENT SCHEDULES CONTAINED IN PART IV OF THIS FORM 10-K CONSTITUTE THE ANNUAL REPORT WITH RESPECT TO THE TRUST AND INVESTING POOL FOR PURPOSES OF CFTC RULE 4.22(C) (THE “CFTC ANNUAL REPORT”).

EXPLANATORY NOTE

This Amendment No. 1 to the Annual Report on Form 10-K for the year ended December 31, 2011 (the “Amendment”) amends the Annual Report on Form 10-K for the same period originally filed by the iShares^® S&P GSCI^TM Commodity-Indexed Trust (the “Trust”) and the iShares^® S&P GSCI^TM Commodity-Indexed Investing Pool LLC (together, with the Trust, the “Co-Registrants”) on February 29, 2012 (the “Original Filing”). The sole purpose of the Amendment is to correct Exhibit 32.2 of the Original Filing, which initially included a typographical error incorrectly naming the Chief Financial Officer on the certification included in the exhibit. The original certification on behalf of the Co-Registrants had been manually signed by the correct person, acting in his capacity as Chief Financial Officer, prior to the filing of the Original Filing, but the identity of the Chief Financial Officer was incorrectly included in the language of the certification required pursuant to 18 U.S.C. § 1350, as adopted pursuant to § 906 of the Sarbanes-Oxley Act of 2002, included in the Original Filing. Accordingly, the corrected certification is included in this Amendment.

Except as described above, the Amendment does not otherwise modify the information contained in the Original Filing, and the Trust has made no attempt to update such information to reflect changes or events taking place after February 29, 2012.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant and co-registrant have duly caused this report to be signed on their behalf by the undersigned in the capacities* indicated, thereunto duly authorized.

For purposes of CFTC Rule 4.22(h), to the best of the knowledge and belief of the undersigned, the information contained in the CFTC Annual Report set forth herein is accurate and complete.

BlackRock Asset Management International Inc.

Sponsor of the iShares^® S&P GSCI™ Commodity-Indexed Trust (registrant)

Manager of the iShares^® S&P GSCI™ Commodity-Indexed Investing Pool LLC (co-registrant)