The effects of surface segregation on the oxygen reduction reaction (orr) have been studied on a polycrystalline Pt3Ni alloy in acid electrolyte using ultra high vacuum (UHV) surface sensitive probes and the rotating ring disk electrode (RRDE) method. Preparation, modification and characterization of alloy surfaces were done in ultra high vacuum (UHV). Depending on the preparation method, two different surface compositions of the Pt3Ni alloy are produced: a sputtered surface with 75% Pt and an annealed surface (950 K) with 100% Pt. The latter surface is designated as the 'Pt-skin' structure, and is a consequence of surface segregation, i.e. replacement of Ni with Pt atoms in the first few atomic layers. Definitive surface compositions were established by low energy ion scattering spectroscopy (LEISS). The cyclic voltammetry of the 'Pt-skin' surface as well as the pseudcapacitance in the hydrogen adsorption/desorption potential region is similar to that of a polycrystalline Pt electrode. The activities of the orr on Pt3Ni alloy surfaces were compared to that of a polycrystalline Pt in 0.1 M HClO4 electrolyte for the observed temperature range of 293 < T < 333 K. The order of activities at 333 K was: 'Pt-skin' > Pt3Ni (75% Pt) > Pt with the maximum catalytic enhancement obtained for the 'Pt-skin' being 4 times that for pure Pt. Catalytic improvement of the orr on Pt3Ni and 'Pt-skin' surfaces was assigned to the inhibition of Pt-OHad formation (on Pt sites) versus polycrystalline Pt. Production of H2O2 on both surfaces was similar compared to that on the pure Pt. Kinetic analyses of RRDE data confirmed that the kinetic parameters for the orr on the Pt3Ni and 'Pt-skin' surfaces are the same as on pure Pt: reaction order, m = 1, two identical Tafel slopes, activation energy, approximate to 21 -25 kJ mol(-1). Therefore, the reaction mechanism on both Pt3Ni and 'Pt-skin' surfaces is the same as that proposed for pure Pt, i.e. a 4e(-) reduction pathway. (C) 2003 Elsevier Science B.V. All rights reserved.