A CO-tolerant PtRu(x)Sn(y)/C electrocatalyst, with an optimal x/y ratio of 0.8/0.2, was prepared by selectively depositing Sn on the metallic surface of PtRu(0.8)/C for use as the anode in a polymer electrolyte membrane fuel cell. The CO tolerance of the catalyst was greater when Sn was added by chemical vapor deposition (CVD) than by a conventional precipitation method because most of the Sn added by CVD was located in the vicinity of Pt and Ru surfaces, on which CO molecules were strongly adsorbed. Accordingly, the bi-functional mechanism of CO oxidation, which involved the migration of oxygenated species from the Sn to the adsorbed CO, was expected to be promoted to greater extents in the catalysts prepared by Sn-CVD than those prepared by Sn-precipitation. On the other hand, the ligand-effect mechanism of CO oxidation, which was facilitated by the Pt-Ru alloy formation, was not much affected by the added Sn because the Pt-Ru alloy remained unchanged, particularly when y <= 0.2. Among PtRu(x)Sn(y)/C catalysts prepared by Sn-CVD, one prepared by partially substituting Sn for Ru in the PtRu(1.0)/C catalyst, e.g., PtRu(0.8)Sn(0.2)/C, showed higher CO tolerance than one prepared by simply adding Sn to the PtRu(1.0)/C catalyst, e.g., PtRu(1.0)Sn(0.2)/C, which demonstrated the importance of an optimum x/y ratio in the design of the ternary PtRu(x)Sn(y)/C catalysts. Crown Copyright (C) 2010 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.