Molecular oxygen electroreduction at both Pt and Au electrodes in acetonitrile solutions was studied by RDE and cyclic voltammetry techniques. Hydrogen peroxide was detected as the main reaction product on Pt as well as on Au electrodes. Molecular oxygen dissociative adsorption is inhibited by the organic solvent which produces a leveling effect of metal catalytic properties. Results obtained by cyclic voltammetry allowed the interpretation of the electrochemical process as a reversible single electron transfer to give O2− , followed by the disproportionation of superoxide ion that in the presence of residual water could account for hydrogen peroxide formation. The disproportionation constant was evaluated, its value being of the order of 105 M sec−1. These results were further confirmed by the experiments performed at the RDE.