Exhibit 8.1

[Form of Tax Opinion of Cravath, Swaine & Moore LLP]

May 1, 2007

State Street Corporation

One Lincoln Street

Boston, Massachusetts 02111

Re: Agreement and Plan of Merger dated as of February 4, 2007 by and between State Street Corporation and Investors Financial Services Corp.

Ladies and Gentlemen:

We have acted as counsel for State Street Corporation, a Massachusetts corporation (“State Street”), in connection with the Agreement and Plan of Merger dated as of February 4, 2007 (the “Merger Agreement”) by and between State Street and Investors Financial Services Corp., a Delaware corporation (“Investors Financial”), pursuant to which Investors Financial will merge with and into State Street (the “Merger”) with State Street being the surviving corporation, on the terms and conditions set forth therein. For purposes of this opinion, capitalized terms used and not otherwise defined herein shall have the meaning ascribed thereto in the Merger Agreement and references herein to the Merger Agreement shall include all exhibits and schedules thereto.

We have examined (i) the Merger Agreement, (ii) Registration Statement on Form S-4 (as amended through the date hereof, the “Registration Statement”), and (iii) the representation letters of State Street and Investors Financial delivered to us for purposes of this opinion (the “Representation Letters”). In addition, we have examined and relied as to matters of fact upon, originals or copies, certified or otherwise identified to our satisfaction, of such corporate records, agreements, documents and other instruments and made such other inquiries as we have deemed necessary or appropriate to enable us to render the opinion set forth below. In such examination, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies, and the authenticity of the originals of such latter documents. We have not, however, undertaken any independent investigation of any factual matter set forth in any of the foregoing.

In rendering such opinion, we have assumed, with your permission, that (i) the Merger will be effected in accordance with the Merger Agreement, (ii) the relevant statements concerning the Merger set forth in the Merger Agreement and the Registration Statement are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time, (iii) the representations made by State Street and Investors Financial in their respective Representation Letters are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time, and (iv) any representations made in the Merger Agreement, if relevant, or the Representation Letters “to the best knowledge of,” or based on the belief of State Street or Investors Financial or similarly qualified are true, complete and correct and will remain true, complete and correct at all times up to and including the Effective Time, in each case without such qualification. We have also assumed, with your permission, that the parties have complied with and, if applicable, will continue to comply with, the relevant covenants contained in the Merger Agreement. If any assumption above is untrue for any reason, our opinion might be adversely affected and may not be relied upon.

Based upon the foregoing, it is our opinion that for U.S. federal income tax purposes, the Merger will constitute a reorganization within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended (the “Code”), and each of State Street and Investors Financial will be a party to the reorganization within the meaning of Section 368(b) of the Code.

We express our opinion herein only as to those matters specifically set forth above and no opinion should be inferred as to the tax consequences of the Merger under any state, local or foreign laws, or with respect to other areas of U.S. federal taxation. We are members of the Bar of the State of New York, and we do not express any opinion herein concerning any law other than the federal law of the United States.

Our opinion is based on current statutory, regulatory and judicial authority, any of which might be changed at any time with retroactive effect. We disclaim any undertaking to advise you of any subsequent changes of the matters stated, represented or assumed herein or any subsequent changes in applicable law, regulations or interpretations thereof.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement, and to the references to our firm name under the headings “Material United States Federal Income Tax Consequences of the Merger” and “Legal Matters” in the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended or the rules and regulations of the Securities and Exchange Commission thereunder.

Very truly yours,