A series of xAl-(1-x)Ce oxides (x=0-0.20) were prepared as supports by the coprecipitation method. 1 wt% Pt was impregnated on the Al-modified Ce oxide supports, which were tested as catalysts for CO oxidation in the absence and presence of H2O vapor. The prepared catalysts were characterized by X-ray diffraction (XRD), N2 sorption, CO temperature-programmed reduction (CO-TPR), 27Al magic-angle spinning (MAS) NMR, and CO-chemisorption analyses. Upon comparison of the catalytic results obtained from the 1 wt% Pt/xAl-(1-x)Ce oxide catalysts, the Pt/0.10Al-0.90Ce oxide catalyst was found to exhibit the highest catalytic activity. When water vapor was present in the feed stream, the catalytic activity increased remarkably, and T90% shifted to a temperature ca. 30 ℃ lower compared to that in dry conditions due to the promotion effect by the water-gas shift reaction. The catalytic activity could be correlated with the Pt dispersion and the amount of surface or lattice oxygen.