We explored the synthesis of Ag/SiO2 catalysts via gamma-ray irradiation at room temperature. AgNO3/SiO2 prepared by the conventional incipient wetness impregnation method was employed as the precursor. Ag/SiO2 catalyst prepared by calcining AgNO3/SiO2 at 500 degrees C was also prepared for comparison. The Ag/SiO2 catalysts were characterized with X-ray diffraction, X-ray photoelectron spectroscopy, and diffuse reflectance UV-vis spectrum, and their catalytic activities towards CO oxidation were also tested. gamma-Ray irradiation successfully reduced Ag+ anchored on SiO2 into silver nanoparticles at room temperature. Existence of excess water during the gamma-ray irradiation led to the formation of larger silver nanoparticles. These catalysts behaved differently in CO catalytic oxidation. Compared with Ag/SiO2 prepared by calcination that showed a certain low temperature catalytic activity but poor stability, Ag/SiO, prepared by room temperature gamma-ray irradiation showed an inferior low temperature catalytic activity and a better stability. The different catalytic behavior is associated with the various crystalline sizes of silver nanoparticles in Ag/SiO2 prepared by different methods. (c) 2007 Elsevier B.V. All rights reserved.