EXHIBIT 23 CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM We hereby consent to the incorporation by reference in the Registration Statements on Form S-3 (No. 333-64787) and on Form S-8 (Nos. 333-03683, 333-19103, 333-19099, 333-82353, 333-96879 and 333-96881) of Cabot Corporation of our report dated December 14, 2005 relating to the financial statements, management's assessment of the effectiveness of internal control over financial reporting and the effectiveness of internal control over financial reporting, which appears in this Form 10-K. /s/ PricewaterhouseCoopers LLP Boston, Massachusetts December 14, 2005