Pt−Ni alloy nanoparticles supported on carbon black (Pt:Ni = 1:1) were prepared by the borohydride reduction method using acetate anions as a stabilizer in anhydrous ethanol solvent. Here, we surveyed the effect of oxide phases in Pt−Ni alloy nanoparticles on the electrocatalytic activity toward oxygen reduction reaction (ORR). As-prepared Pt1Ni1/C, which showed a relatively high degree of alloying, possessed the lower oxygen reduction reaction (ORR) activity as compared to pure Pt. However, following heat treatment in a flow of Ar at 300 °C for 3 h, Pt1Ni1/C showed oxygen reduction activity higher than that of commercial Pt/C (40 wt % Pt/C, Johnson-Matthey). The potential of zero total charge (PZTC) was calculated from cyclic voltammograms and the CO-displacement charge at dosing potentials at which anions are the main adsorbed species. The calculated value then shifted to a more positive potential after heat treatment. This indicates that the surface of the Pt−Ni nanoparticles became less oxophilic main...