A study of the electroreduction of O-2 (ORR) in 1 M NaOH on a polycrystalline platinum surface pretreated by H-2 evolution at -250 mV in deaerated 0.5 M H2SO4 for 10 min was carried out. As a consequence of this treatment, the Pt surface evolves spontaneously with time toward a Nekrasov (N-Pt) type behavior both in the cathodic and in the anodic scan. We studied the mechanism of the ORR on two N-Pt surfaces we called A and C. On both surfaces, O-2 is reduced to HO2- which is then reduced or disproportionated. On surface A, a full N-Pt type surface, HO2-is reduced only below 170 mV and disproportionates above this potential producing O-2 that is recycled below 600 mV giving rise to an enhancement of the current above the two-electron level. HO2- disprorportionates on surface C as well, although with a greater rate constant, producing O-2 that is also recycled. The details of the electroreduction of O-2 and HO2-are thoroughly discussed in relation to the surface processes underneath revealed by the characteristic cyclic voltammograms of the two surfaces.