Applying quantitative temporal analysis of products (TAP) techniques, we demonstrate that after calcination, a Au/CeO2 catalyst is only slightly active for CO oxidation. The activity is significantly increased on removal of about 7% of the surface oxygen, whereas overreduction leads to initially lower activity. Reductive activation can occur either during reaction, in the (oxidative) reaction gas, or by controlled prereduction. This is the first experimental verification of a CO oxidation rate enhancement by oxygen surface vacancies on a realistic oxide-supported An catalyst. Consequences for the reaction mechanism are discussed. (C) 2007 Elsevier Inc. All rights reserved.