Influence of residual K+ on the preferential oxidation of CO in excess hydrogen (PROX) over CuO-CeO2 catalysts was investigated. CuO-CeO2 catalysts were characterized by BET, ICP, XRD, UV-Raman and TPR techniques. The results showed that the existence of residual K+ made a peak in TPR of CuO-CeO2 catalysts shift to higher temperatures and depressed the PROX in the absence Of CO2 and H2O in the feed over CuO-CeO2 catalysts. However, small amount of residual K+ was beneficial to the catalytic performance of CuO-CeO2 catalysts in the PROX in the presence Of CO2 and H2O in the feed. Consequently, residual K+ with an appropriate content was beneficial to improve the catalytic performance of CuO-CeO2 catalysts in the presence Of CO2 and H2O. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.