A procedure was developed to synthesize a platinum-cobalt (Pt-Co) alloy electrocatalyst for oxygen reduction using Co/C composite as a support. The Pt-Co/C catalysts were synthesized through: (i) chemical oxidation of carbon black, (ii) Co deposition on the oxidized carbon using a chelation method, (iii) chemical treatment in an acidic medium to remove excess of Co on the carbon surface, (iv) Pt deposition onto the Co/C support, and (v) postheat treatment to form the Pt-Co alloy catalyst. The synthesized Pt-Co/C catalyst showed improved activity and long-term stability in polymer electrolyte membrane (PEM) fuel cells when compared with a conventional Pt-Co/C catalyst. The electron probe microanalysis combined with scanning electron microscopy indicated that the Co content in the alloy catalyst remains stable without poisoning of the electrolyte membrane during long-term operation.