An Au/SiO2 system with most of the metal particles less than 2.0 nm in size were prepared by deposition on silica of preformed gold sols derived from Au(PPh3)(3)Cl. In this form, it was inactive in the CO + O-2 reaction at ca. 333 K, because of the presence of phosphine ligands on the surface of metal particles. The system became catalytically active in this reaction after treatment in O-2 at 673 K, which also resulted in a slight sintering of the metal phase (mean size = 2.9 nm). By subsequent heating in H-2 at 673 K, the mean size of gold particles increased up to 3.4 nm. Such particles exhibited a lower amount of step surface sites able to adsorb CO, but became more active in the CO + O-2 reaction. This suggested that the reaction steps involving oxygen can occur more effectively on Au sites present on larger and smoother gold particles.