In this paper we investigated the effect of oxygen treatment on the catalytic performance of Au/SiO2 catalysts towards CO oxidation. The Au/SiO2 catalysts were prepared via the deposition-precipitation method. The as-prepared Au/SiO2 catalyst behaves a poor catalytic activity, which is attributed to the relatively large size of Au particles. Treatment of the catalyst in O-2 at temperatures higher than 800 degrees C greatly enhances the catalytic activity. However, XRD results demonstrate that the oxygen treatment at temperatures above 800 degrees C leads to the agglomeration of Au particles. Meanwhile, TEM results reveal the co-existence of well-dispersed ultrafine An particles on the surface. XPS results reveal that the Au 4f binding energy shifts to higher binding energy after the oxygen pretreatment at temperatures above 800 degrees C. Interestingly, treatment of the Au/SiO2 catalyst in He at 800 degrees C also shows the similar geometric and electronic structure changes of Au particles and thus the enhancement effect as that in O-2, indicating that the Au-O-2 interactions at high temperatures do not contribute much to the improvement of catalytic activity. We propose an evaporation-deposition mechanism of gold particles treated at high temperatures, accounting for the formation of ultrafine Au particles, which are responsible for the enhancement of the catalytic activity. (c) 2006 Elsevier B.V. All rights reserved.