Cabot (CBT) 10-Q 2012 Q3 Quarterly report

EXHIBIT 18.1

August 6, 2012

Cabot Corporation

Two Seaport Lane

Boston, Massachusetts

Dear Sirs/Madams:

At your request, we have read the description included in your Quarterly Report on Form 10-Q to the Securities and Exchange Commission for the quarter ended June 30, 2012, of the facts relating to the change in the annual goodwill impairment testing date from March 31 to May 31. We believe, on the basis of the facts so set forth and other information furnished to us by appropriate officials of the Company, that the accounting change described in your Form 10-Q is to an alternative accounting principle that is preferable under the circumstances.

We have not audited any consolidated financial statements of Cabot Corporation and its consolidated subsidiaries as of any date or for any period subsequent to September 30, 2011. Therefore, we are unable to express, and we do not express, an opinion on the facts set forth in the above-mentioned Form 10-Q, on the related information furnished to us by officials of the Company, or on the financial position, results of operations, or cash flows of Cabot Corporation and its consolidated subsidiaries as of any date or for any period subsequent to September 30, 2011.

Yours truly,

/s/ Deloitte & Touche LLP

Boston, Massachusetts