The details of the preparation and characterization of co-deposited Pt-Ru catalysts supported by highly oriented pyrolytic graphite are presented. The Pt-Ru catalysts were prepared by changing applied potentials in chloroplatinic and ruthenium solution of 20 mM, respectively. Electrocatalytic activity for methanol oxidation reaction of Pt-Ru catalysts prepared by applying a potential of -0.25 V was better than that of catalysts prepared at other applied potentials. This indicates that there is an optimum potential for electrodeposition of Pt and Ru. The activation energy of Pt-Ru catalysts is calculated as 38 and 47 kJ/mol in the 0.3 and 0.4 V, respectively. To investigate the stability of Pt-Ru catalysts, Ru stripping on Pt-Ru and Ru/Pt disk electrodes was investigated by changing the cycling numbers of cyclic voltametry and holding time at constant potential, respectively. The peak potential of methanol oxidation is shifted to high potential by increasing the cycling numbers and holding time. Shifts of peak potential are discussed in terms of the oxidation pathway of CO adsorbed on the surface of Pt-Ru catalyst. The particle size of Pt-Ru catalyst measured by SEM is about 75 Angstrom and the range of particle size measured by STM is from 35 to 110 Angstrom. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.