Abstract. Platinum-nickel alloys have been considered one of the promising alternative catalysts for replacement of Pt since they show higher reactivity with less usage of Pt. To elucidate the role of Ni in the various stages of the oxygen reduction reaction and the corresponding changes to Pt in comparison with pure Pt clusters, we have conducted Pt L₃ and Ni K edge in situ XAFS measurements on carbon supported Pt and PtNi(1:1) nanoparticle catalysts under a wide range of operating potentials. We observe that in PtNi alloys the Pt-Pt bond distance is shorter and the distribution of Pt and Ni is non-uniform: Pt has a tendency to be found on the surface while Ni is mostly in the interior of the catalyst nanoparticles. In addition, while a change in oxidation of the pure Pt nanoparticles is clearly observed at different potentials, the Pt in the PtNi alloy remains nearly oxygen-free at all potentials but an accompanying oxidation change of Ni has been observed instead. This phenomenon suggests that the presence of Ni in the nanoparticle inhibits the coverage of oxygen adsorbate on Pt surface, resulting in more active sites on the Pt surface.