To investigate the enhancement of the oxygen reduction reaction (ORR) activity on a carbon-supported PtAu alloy nanoparticle catalyst, the catalyst was subject to heat treatment at 423 K under a CO or Ar atmosphere. The surface composition was analyzed by XPS and the composition was shown to increase from 66 +/- 2% (PtAu-AP) to 74 +/- 2% ( PtAu-CO) after heat treatment under a CO atmosphere, which was confirmed by electrochemical techniques, while the bulk composition was invariant at 55%. For the oxygen reduction reaction (ORR), the mass activity of PtAu-CO increased by 75.6% (33.2 A/g(pt)) when compared to that of PtAu-AP (18.9 A/g(pt)). Since the increase in EAS was only 15.8%, it was concluded that the specific activity was enhanced by 52.6% due to surface Pt segregation after heat treatment under a CO atmosphere. The enhanced specific activity was attributed to the reduced OH adsorption energy which was characterized by measuring the potential of total zero charge. The weaker OH adsorption was resulted from the higher Pt/Au ratio at the surface layers. (C) 2012 Elsevier B.V. All rights reserved.