Carbon-supported PtCo nanoparticles catalysts with core-shell structure were prepared via reduction method and subsequent electroless deposition. It was found that pH has critical effect on the morphology and activity of the synthesized catalyst. The favorable pH to prevent the formation of oxide impurities and extensive Co dissolution during the replacement reaction is neutral. The intrinsic activity of the PtCo/C catalysts for oxygen reduction reaction was found to be 2-4 times higher compared with that of Pt/C. This activity enhancement may originate from the favorable Pt-Pt interatomic distance and low defect that would inhibit the formation of Pt-OHad at low overpotential. (C) 2010 Elsevier Ltd. All rights reserved.