We present a novel efficient catalyst, Au-Ag alloy nanoparticles Supported oil mesoporous aluminosilicate, The catalysts were applied to the low-temperature CO oxidation reaction. The sample was prepared in one pot. in which the formation of nanoparticles was coupled in aqueous solution with the construction of mesoporous Structure. Both XRD and TEM characterizations show that the alloy particles are much larger than the monometallic gold particles and become even bigger with an increase in the amount of Ag. We shall demonstrate that such large particles with an average particle size of about 20-30 nm exhibit exceptionally high activity for CO oxidation it low temperatures, Moreover, the activity varies with the Au/Ag molar ratios and attains the best conversion when Au/Ag is 3:1. The presence of excess H-2 deactivates the alloy activity completely at room temperature. UV-vis and EXAFS confirm the Au-Ag alloy formation, XPS results show that the alloy catalysts are in the metallic state, and they have a greater tendency to lose electrons than do the monometallic catalysts. EPR results show there is an O-2(-) species on the catalyst surface, and the intensity of the O-2(-) species becomes the strongest at Au/Ag = 3: 1. The catalytic activity coincides with the magnitude of O-2(-) EPR signal intensities. Based on the spectroscopic study and catalytic activity measurements, a reaction mechanism has been proposed. (c) 2005 Elsevier Inc. All rights reserved.