Cabot (CBT) 8-K Changes in Registrant.s Certifying Accountant

December 22, 2006

Securities and Exchange Commission
100 F Street, N.E.
Washington, DC 20549

Commissioners:

We have read the statements made by the Cabot Retirement Savings Plan, a benefit plan sponsored by Cabot Corporation, (copy attached), which we understand will be filed with the Securities and Exchange Commission, pursuant to Item 4.01 of Form 8-K, as part of the Form 8-K of Cabot Corporation dated December 19, 2006. We agree with the statements concerning our Firm in such Form 8-K.

Very truly yours,

/s/ PricewaterhouseCoopers LLP