Electrochemical oxygen reduction on a platinum electrode has been investigated in methanol-0.05 M H2SO4 aqueous solutions in the presence of acids of cobalt bis(dicarbollide) (H+B-, H+[(1,2-C2B9H11)(2)-3-Co]((-))) or its hexachlorinated derivative (H+BCl-, H+ [(8'8',9,9',12, 12'-Cl-6-(1,2-C2B9H8)(2)-3-Co)]((-))). Methanol oxidation was Suppressed with H+BCl- significantly, and selective oxygen reduction was achieved on a platinum electrode. The efficiency of oxygen reduction to H2O, which depended on the electrode potential, was lower by 2% to 20% than that without H+BCl-. The mechanism was investigated by surface-enhanced infrared absorption spectroscopy (SEIRAS). H+BCl- suppresses formation of both the catalytic poison, linear CO (ads.), from methanol and the formate, the intermediate of the non-CO path of methanol oxidation. (C) 2005 Elsevier Ltd. All rights reserved.