The chemical environment of Ag and the catalytic activity for CO oxidation with O-2 of three kinds of bimetallic catalysts, i.e., Ag contaminated Au powder, nanoporous gold (NPG), and Ag-Au colloidal NPs, were examined in order to get insight into the nature of active sites for CO oxidation and the genesis of the catalytic activity for CO oxidation. Although the preparation method and the morphology of three Au Ag bimetallic catalysts are largely different one another, the enrichment of Ag in the surface layer could be observed on any of three Au-Ag bimetallic catalysts, and the Arrhenius plots of the rate constant of CO oxidation per unit metallic surface area, i.e., km,, determined on three bimetallic catalysts showed almost the same slope. The Ag-K edge EXAS analysis performed on three bimetallic catalysts revealed that the chemical environments of Ag atoms contained in three bimetallic catalysts are similar one another: (I) Ag atoms are surrounded by Ag-Ag and by Ag-Au bonds and (2) the CN (coordination number) of Ag-Au is larger than that of Ag-Ag, indicating that Ag is highly dispersed in the surface layer of three bimetallic catalysts. The CO oxidation on three bimetallic catalysts is considered to proceed according to the same mechanism on active sites with similar structure consisting of Ag and Au. The extent of the dispersion of Ag in the surface layer seemed to correlate with the values of k(CO2) determined for the three AgAu bimetallic catalysts. (C) 2014 Elsevier B.V. All rights reserved.