The proton exchange membrane fuel cells with potentially much higher efficiencies and almost zero emissions offer an attractive alternative to the internal combustion engines for automotive applications. A critical issue in fuel cell system is the small (similar to0.5%) amount of CO present from output of fuel reformer. This amount of CO can deteriorate the performance of fuel cells. Consequently, an additional gas conditioning process is required to minimize CO content in reformed gas. The proposed research study focuses on preferential oxidation of CO in a simulated reformed gas to CO2 by using selective CO oxidation catalysts. In this work, the effects of preparation method, O-2, water vapor, and CO2 concentration in feed stream on the selective CO oxidation over Au/CeO2 catalysts were investigated in the temperature range of 323-463 K. Catalytic stability test was also performed. We find that the activity of Au catalyst depends very strongly upon the preparation method, with co-precipitation prepared Au/CeO2 catalyst exhibiting the highest activities. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.