The oxygen reduction reaction (ORR) on carbon-supported Pt-Co alloy nanoparticles has been investigated. As-prepared Pt-Co alloy nanoparticles exhibit a single-phase disordered structure and have a very small particle size. Such alloy catalysts exhibit an enhancement of a factor of ca. 1.3-3.2 in Pt mass activity and of a factor of ca. 1.2-2.2 in the specific activity for the ORR when compared to Pt/C catalyst. The enhanced electrocatalytic activity is attributed to the favorable Pt-Pt mean interatomic distance caused by alloying and to the high dispersion of the alloy catalysts. (c) 2006 Elsevier B.V. All rights reserved.