Carbon supported Pt85Co15 electrocatalysts for the oxygen reduction reaction (ORR) in PEM fuel cells were prepared by reduction of Pt and Co precursors with sodium borohydride in acid and alkaline media at room temperature. The clectrocatalyst prepared in the acidic conditions showed better kinetic parameters for the ORR than the electrocatalyst prepared in basic conditions. This result was ascribed to a higher degree of alloying, with a shortening of the Pt-Pt bond distance, of the sample prepared in acid medium. At high current densities, instead, the performance in PEM fuel cells of the carbon supported alloy prepared in acid medium was lower than that prepared in basic medium, due to the larger metal particle size. (C) 2004 Elsevier B.V. All rights reserved.