Factors influencing basic catalytic activity of Ag/SiO2 catalysts for CO selective oxidation in H-2-rich gases, such as support, silver loading, oxygen concentration and reaction temperature, etc. were investigated. Among the supports used, SiO2 and the pure silica zeolite are better supports for silver catalyst. Ag/SiO2 catalyst has a better stabilization for CO selective oxidation at low temperatures. Carbon monoxide in the presence of large excess of hydrogen can be stoichiometrically oxidized by oxygen over Ag/SiO2 catalysts at low temperatures, and the catalytic activities increase with the increase of the silver loading. Low reaction temperature favors this process of selective oxidation of CO in H-rich gas, and the selectivity Of O-2 to oxidize CO can reach 60-80%. The maximum conversion of CO under such selectivity levels for oxygen to oxidize CO also increases with the silver loading. XRD results show that the main species on all catalysts is crystallite silver, and the size of the silver particles increases with the silver loading. It is found that the CO conversion increases with increasing oxygen contents properly, however, CO is not consumed completely even when the overall concentration Of O-2 is increased to 3%. Pretreatment conditions have been found to strongly influence the activity and selectivity of the catalysts. The catalysts deactivate after pretreatment with hydrogen at 500 degrees C, whereas a better activity is obtained after pretreatment with oxygen or He at the same temperature. (c) 2005 Published by Elsevier B.V.