An Ir-in-CeO2 catalyst was activated by pulses of CO, and the effects of the formed oxygen vacancy and carbonates on CO oxidation were studied by a pulse calorimetry. A volcano-like relationship between the amount of oxygen vacancy and the catalyst activity is established. The presence of oxygen vacancy is favorable for CO oxidation, with the best activity when 1.5% amount of oxygen vacancy is obtained. However, the activity decreases with further increase of oxygen vacancy. A quantitative analysis of the reaction process suggests that the increase of oxygen vacancy promotes the buildup of strongly adsorbed carbonates, leading to the prohibition of the adsorption and activation of CO but not O-2. The reduced supply of adsorbed CO to react with oxygen species may result in the decreasing activity with more oxygen vacancy. (C) 2011 Elsevier B.V. All rights reserved.