The electrocatalytic reduction of oxygen on carbon-supported Pt-Cr (1:1) alloy nanoparticle catalysts with two different metal loadings prepared via a carbonyl route was investigated based on the porous thin-film rotating disk-ring electrode technique and compared with that on E-TEK Pt/C catalyst in pure and methanol-containing electrolytes. The as-prepared Pt-Cr alloy nanoparticles, which have single-phase disordered structures, are well dispersed on the surface of carbon with a narrow size distribution even at 40 wt % metal loading. Such catalysts are stable in air up to 600 degrees C. As compared to the Pt/C catalyst, the alloy catalysts showed slightly enhanced activity for the oxygen reduction reaction in pure acid electrolyte and significantly enhanced activity in the presence of methanol, and the ring-current measurements on the homemade catalysts showed a reduction in peroxide yield in pure acid solution. The enhanced activity could be ascribed to the effect of alloying on the initiation and extent of surface oxide formation. Oxygen reduction kinetic analysis indicated a potential dependence of the apparent number of electrons transferred per oxygen molecule during the reduction in methanol-containing solution. High methanol tolerance of Pt-Cr alloy catalysts during the oxygen reduction could be explained well by the lower reactivity of methanol oxidation, which may originate from the composition effect and the dAll rights reserved.ordered structure of the alloy catalysts. (c) 2005 The Electrochemical Society.