The electrochemical oxidation of COad was investigated on smooth and high area platinum, platinum-tin and platinum-ruthenium electrodes. On high area platinum, COad was classified into two types by their oxidation potentials : an easily oxidized one and one which is hard to oxidize. Pt-Ru was found to promote the oxidation of the latter, whereas Pt-Sn did the same for the former. The ternary system, Pt-Ru-Sn, showed even higher catalytic activity for COad oxidation than the binary systems.