Pt catalysts supported on a series of CrxFe2-xO3 composite oxides with different Cr/Fe molar ratios were prepared and tested for CO oxidation. The Pt/Cr1,3Fe(0.7)O(3) catalyst possesses the best performance (with a turnover frequency of ca. 0.2 s(-1) at 80 C under 1% CO + 1% 02 condition), due to its best reducibility which was related to the solid solution structure of the support. The catalyst also shows good stability in the presence of both 10% CO2 and 10% H2O. Detailed kinetic results and staged reaction indicate that CO2 and H2O play opposite roles in the reaction. The inhibiting role of CO2 is due to competitive adsorption of CO2 with CO and the formation of carbonate species. However, the addition of H2O could effectively decompose the carbonate and sustain the catalyst stability. Moreover, the promoting role of H2O could be mainly attributed to the weakened strength of CO adsorption and the fast interfacial reaction between CO and the surface hydroxyl groups formed via H2O dissociation, as revealed by the X-ray photoelectron spectroscopy, CO temperature-programmed desorption and the kinetic results.