Sn-Pt/SiO2 catalysts prepared by using controlled surface reactions are highly active in the low-temperature oxidation of CO. The results indicate that the high activity of these catalysts can be attributed to (i) the stabilization of tin-platinum alloy phases on the silica support, and (ii) the formation of "Sn4+-Pt" ensemble sires during the oxidation of CO. It is suggested that Sn4+ species formed in situ are involved in the activation of the CO molecule adsorbed at the neighboring Pt site. This activation strongly resembles that of the carbonyl group in unsaturated aldehydes over this type of Sn-Pt/SiO2 catalysts.