In order to elucidate the roles of pretreatment with oxygen at high temperatures and the interaction between Ag nanoparticles and SiO2 in low-temperature CO selective oxidation, a mechanical mixture of Ag nanoparticles and SiO2 powder was prepared and pretreated with oxygen at different temperatures. The samples calcined at 300 and 400 degreesC have poor activities for CO oxidation, while oxygen reacts totally with H-2 at temperatures of 160 and 120 degreesC, respectively. It becomes catalytically active after treatment with oxygen at 500 degreesC, and the activity increases with the treatment temperature up to 700 degreesC, reaching nearly the same level of CO conversion as that of the 8%Ag/SiO2 prepared by the incipient wetness impregnation method and treated with He at 500 degreesC. XRD and TEM results show that calcinations under oxygen atmosphere between 500 and 700 degreesC help dispersing the silver nanoparticles on the surface Of SiO2, as well as induce an interaction between the silver nanoparticles and SiO2. Moreover, the ability of CO adsorption at low temperatures is obviously enhanced. However, further increasing the pretreatment temperature decreases the catalytic activity due to the heat-induced aggregation of silver nanoparticles on the surface Of SiO2 and the corresponding decrease of the CO adsorption ability, but higher than that of the initial Ag-SiO2 mixture. CO adsorbed on the Ag nanoparticles dispersed on the surface of silica shows a high activity toward CO selective oxidation, and it is proved that the sub-surface oxygen species plays a very important role in this reaction. (C) 2004 Published by Elsevier B.V.