Carbon-supported Pd-Co bimetallic nanoparticle electrocatalysts of different Pd/Co atomic ratios were prepared by a modified polyol reduction. Electrocatalytic activities of the catalysts for the oxygen reduction reaction (ORR) have been investigated based on the porous rotating disk and disk-ring electrode techniques. As-prepared Pd-Co bimetallic nanoparticles evidence a single-phase fec disordered structure, and the mean particle size is found to decrease with increase in Co content. A typical TEM image of the Pd2Co/C catalyst, heat-treated at 500 degrees C, reveals a mean particle diameter is ca. 8.3 nm with a relatively narrow size distribution. For synthesized Pd-Co catalysts, the highest catalytic activity for the ORR, when supported on carbon (i.e., Pd-Co/C) was found for a Pd:Co atomic ratio of 2:1 and heat treatment at ca. 500 degrees C, corresponding to a Pd-Pd mean interatomic distance of ca. 0.273 nm. Kinetic analysis based on the rotating disk and disk-ring electrode measurements reveals that the ORR on Pd-Co/C catalysts undergoes a four-electron process in forming water Because the Pd-Co/C catalyst is inactive for the adsorption and oxidation of methanol, it may function as a methanol-tolerant ORR catalyst in a direct methanol fuel cell. (c) 2007 Elsevier B.V. All rights reserved.