The light-off temperature of the oxidations of CO and CA over a commercial three-way catalyst.(TWC) was shifted to a lower temperature by the addition of water to the feed stream. From FTIR analysis, the formation of carboxylate and carbonate by a reaction between adsorbed CO and OH on the catalyst surface was observed during the course of the reactions. In addition, the CO desorption peak of CO-TPD shifted to a lower temperature by water, which may be directly indicative of the reduction of the adsorption strength of CO on the catalyst surface. Detailed reaction kinetic models have been developed in view of the concept of microkinetics to describe the effect of water, particularly on the oxidation activity over a commercial TWC. (C) 2007 Elsevier B.V. All rights reserved.