We report a comparative study of the alloy formation and electrochemical activity of dealloyed PtCo3 and PtCu3 nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). For the Pt-Co system the maximum annealing temperatures were 650 degrees C, 800 degrees C and 900 degrees C for 7h to drive the Pt-Co alloy formation and the particle growth. EDS and XRD were employed for the characterization of catalyst powders. The RDE and RRDE experiments were conducted in 0.1 M HClO4 at room temperature. We demonstrate that the mass and surface area specific ORR activities of Pt-Co and Pt-Cu alloys after voltammetric activation exhibit a considerable improvement compared to those of pure Pt/C. The dealloyed PtCo3 (800 degrees C/7 h) electrocatalyst performs 3 times higher in terms of Pt-based mass activity and 4-5 times higher in terms of ECSA-based specific activity than a 28.2 wt.% Pt/C. Dealloyed Pt-Co catalysts (800 degrees C/7 h) show the most favorable balance between mass and specific ORR activity with a particle size of 2.2 +/- 0.1 nm. We hypothesize that geometric strain effects of the dealloyed Pt-Co nanoparticles, similar to those found in dealloyed PtCu3 nanoparticles, are responsible for the improvement in ORR activity [1]. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.