The active species and particle size of silver catalysts for CO oxidation were discussed in this paper by investigating the effects of Ag loading and different pretreatments on the crystallite structure of silver catalysts. It was found that 8 wt% Ag/SiO2 catalyst pretreated with O-2 at 500 degrees C exhibited high catalytic performance (T-98 = 65 degrees C).When the catalyst further treated with H-2 at 200 degrees C following O-2 treatment, a better catalytic performance (T-98 = 50 degrees C) was obtained. However, it was decreased with the increase the reduction temperature to 300 degrees C. Crystallite Ag and Ag2O species were observed after O-2 pretreatment at 500 degrees C. Subsequent H-2 treatment at low temperatures (50-200 degrees C) caused the reduction of Ag2O species, the formation of Ag-n(delta+) clusters and the high dispersion of crystallite silver. Reduction at high temperature (>200 degrees C) resulted in decreased amount of the oxygen species and the aggregation of metallic silver particles, which were confirmed by the disappearance of the Ag-n(delta+) clusters and the sharpening of the band centered at 410 nm. Therefore, oxygen-containing Ag species might be proposed as the active species and silver particle size of ca. 4.5-5.5 nm would be favorable for the low-temperature CO oxidation. (C) 2010 Elsevier B.V. All rights reserved.