Carbon supported Pt and Pt-Co nanoparticles were prepared by reduction of the metal precursors with NaBH4. The activity for the oxygen reduction reaction (ORR) of the as-prepared Co-containing catalyst was higher than that of pure Pt. 30 h of constant potential operation at 0.8 V, repetitive potential cycling in the range 0.5-1.0V and thermal treatments were carried out to evaluate their electrochemical stability. Loss of non-alloyed and, to a less extent, alloyed cobalt was observed after the durability tests with the Pt-Co/C catalyst. The loss in ORR activity following durability tests was higher in Pt-Co/C than in Pt/C, i.e. pure Pt showed higher electrochemical stability than the binary catalyst. The lower stability of the Pt-Co catalyst during repetitive potential cycling was not ascribed to Co loss, but to the dissolution-re-deposition of Pt, forming a surface layer of non-alloyed pure Pt. The lower activity of the Pt-Co catalyst than Pt following the thermal treatment, instead, was due to the presence of non-alloyed Co and its oxides on the catalyst surface, hindering the molecular oxygen to reach the Pt sites. (c) 2008 Elsevier BY. All rights reserved