A variety of perovskite-type mixed oxides corresponding to the solid solution Ba-12[BaxPt3-x]Pt6O27 (0 less than or equal to x less than or equal to 3) was studied by X-ray photoelectron spectroscopy (XPS). Pt was found to be in two valence states, Pt2+ and Pt4+ and the quantity of Pt2+ decreases when x increases. The intrinsic activity, for catalytic CO oxidation, was found to be dependent on x. At low temperature (below 170 degrees C), the highest activity is obtained for Ba12Pt9O27 (x = 0) and the activity decreased with increasing x. The apparent activation energies increased, from 80 to 110 kJ.mol(-1), when x increased; for a sample of metallic platinum catalyst studied in the same conditions, a value of 120 kJ.mol(-1) was found. The partial reaction order/O-2 (in the range of 0.64 to 0.90) increased with x while the partial order/CO (in the range of -0.34 to -0.70) decreased when x increased. Moreover, for the oxide containing only Pt4+ (Ba15Pt6O27) the reaction orders were similar to those determined for the metallic platinum. Hence, there is undoubtedly a correlation between the Pt2+/Pt4+ ratio in these oxides and the catalytic activity for CO oxidation. This can be explained by assuming that CO chemisorption on Pt2+ is weaker than on Pt4+, which implies a less inhibitor effect of CO at low temperature. (C) 2000 Elsevier Science Ltd.