Tin oxide supported platinum catalysts (Pt/SnOx) for use in polymer electrolyte fuel cells were prepared, and their electrochemical oxidation of CO was investigated. From the XPS measurements of Pt/SnOx catalysts, it was found that the binding energy of Pt showed no change compared to bulk Pt, regardless of tin oxide addition. In accordance with this result, CO-stripping voltammograms for Pt/SnOx catalysts revealed that the peak potential for the pre-adsorbed CO oxidation was almost the same value of Pt/C catalyst. However, the onset potential of CO oxidation shifted negatively compared to PtRu/C catalyst, indicating an enhancement of CO tolerance of Pt/SnOx catalysts. (c) 2005 Elsevier B.V. All rights reserved.