The mechanism of CO oxidation with O-2 on Ag-contaminated Au fine powder (Ag/Au) was studied by kinetic measurements combined with the determination of the amount of CO2 produced. The kinetic behavior of "Ag/Au having the steady state surfaces" indicated that the oxidation proceeds through co-adsorption of CO and O-2 to form (CO3)* succeeded by its decomposition enhanced by the interaction with CO and O-2 to produce CO2 at the contact interface between surface silver and gold support. The kinetics measurements performed on "Ag/Au having the oxidized surfaces" indicated that only the reduction of reactive adsorbed oxygen with CO in the reaction mixture to produce CO2 proceeds at first without incorporation of O-2 in the gas phase and that the participation of O-2 in the CO2 production process as well as its incorporation into the catalyst surface starts gradually as the reduction of reactive adsorbed oxygen on Ag proceeds. (C) 2009 Elsevier Inc. All rights reserved.