Carbon supported Pt and Pt70Co30 electrocatalysts for the oxygen reduction reaction (ORR) were prepared by reduction with formic acid and tested in polymer electrolyte fuel cells. In the presence of Co an increase of the Pt particle size was observed in the as-prepared electrocatalyst and no evidence of Pt-Co alloy formation was detected from XRD measurements. Following thermal treatment (17F) at 900degreesC of the Pt70Co30/C electrocatalyst, the presence in the XRD pattern of secondary Pt reflexions shifted to higher angles indicated partial alloy formation. The fuel cell performance with the as-prepared Pt70Co30/C electrocatalyst was inferior than that with Pt/C. The electrocatalytic activity increased with a TT of the binary electrocatalyst, and the value of the mass activity of the Pt70Co30/C electrocatalyst thermally treated at 900degreesC was only slightly lower than that of Pt/C, notwithstanding the larger metal particle size, about five times that of pure Pt. On the other hand, there was a remarkable increase of the specific activity for the ORR of the Co-containing catalyst after TT at 900degreesC with respect to Pt alone, which was ascribed to both the increased metal particle size and alloy formation. At high current densities the performance of PEMFC electrodes decreased with increasing Pt particle size. (C) 2004 Elsevier B.V. All rights reserved.