Platinum-Palladium supported on ceria catalysts has been studied for preferential oxidation of CO in the presence of hydrogen. The catalysts were prepared by impregnation method using different supports. The catalytic activity tests were performed using a fixed bed reactor in the temperature range of 50-190 degrees C atmospheric pressure. Effects of support, ratio of Pt-Pd, O-2 concentration water vapor concentration, CO2 concentration, and the combination of H2O and CO2 in the feed stream on the catalytic performance of Pt-Pd/CeO2 catalysts were also investigated. The experimental results showed that 1% (1:7) Pt-Pd/CeO2 (sol-gel) exhibited higher activity than other catalysts examined. Water vapor in the feed substantially increased the activity of the catalysts, whereas CO2 in the feed stream drastically decreased the activity of the catalysts. When both H2O and CO2 were added in the feed stream, the positive effect of H2O is more pronounced than the negative effect Of CO2. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.