CoOx/CeO2 composite catalysts of different cobalt/ceria ratios have been prepared and tested for carbon monoxide oxidation in mixtures of carbon monoxide and oxygen. The small activity decay observed for them is due to carbon dioxide retention. The CoOx/CeO2 composite catalyst showed good resistance to water vapor poisoning. The catalysts were investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR), CO-TPD and XPS. The CoOx/CeO2 composite catalysts exhibit high catalytic activity in carbon monoxide oxidation, showing markedly enhanced catalytic activities due to the combined effect of cobalt oxide and ceria. Combining the results of XRD, TPR and XPS, we propose that the finely dispersed and higher valence state CoOx species mainly contribute to the catalytic activity. (C) 2003 Elsevier B.V. All rights reserved.