The performance of PtCo/C electrocatalysts in the oxygen reduction reaction is enhanced after thermal treatment in hydrogen. In fact, the intrinsic activity (per gram of Pt) of PtCo/C electrocatalyst after the adequate treatment is far superior to that of the commercial sample. The PtCo nanoparticles were prepared from the water-in-oil microemulsion technique. The total metal loading of the catalyst was 30 wt%, and two reduction temperatures, 300 and 875 degrees C, were studied. Electrochemical measurements were carried out using the rotating disk electrode method in 0.5 M H2SO4 at room temperature, while the Pt real surface area was determined by CO stripping voltammetry. Analyses from XPS and TPR revealed that the amount of Co and Pt reduced species as well as the particle size (XRD), increased with the thermal treatment. Results derived from the electrochemical analyses were in agreement with those obtained in a H-2/O-2 single cell. These results demonstrate the important role of the cobalt as well as the reduction temperature and atmosphere, and open new ways for the design of improved bimetallic catalyst. (C) 2007 Elsevier B.V. All rights reserved.