The performance of carbon-supported platinum-osmium electrocatalysts (PtOs/C) on the electrocatalysis of the hydrogen oxidation reaction in the presence of CO was studied as a function of the composition and preparation conditions, for materials treated at different temperatures and atmospheres. Anodic polarization curves were obtained with different partial pressures of CO in the H-2 flow. Results show a strong influence of the preparation method in the composition and particle size of the electrocatalysts. At low overpotentials the performance of PtOs/C prepared using formic acid as reducing agent and treated at 500degreesC under H-2 is higher than that of the state-of-the-art PtRu/C electrocatalyst. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.