The oxygen reduction reaction on stainless steels (SSs) was studied in natural seawater with a rotating disk electrode and a rotating ring-disk electrode. Different surface treatments have been performed: prereduction, polishing, passivation and chemical treatment. X-ray photoelectron spectroscopy has been used to characterize the oxide films. The processes of O2 reduction are markedly influenced by the surface oxides and the reduction rate increases in the following order: chemically treated surface<passivated surface<mechanically polished surface<prereduced surface. An effect of the content of iron (FeII) in the oxide films and of Ni are suggested. On prereduced surfaces, O2 is reduced mainly through the four electron pathway, whereas the 2e- and 4e- occur on polished SS. On prereduced and on polished surfaces, O2 reduction is limited by the mass transport in the solution. On passivated surfaces, the oxygen reduction is not limited by the mass transport in the solution. It may be due to a limited access of oxygen to the metal surface and/or to a modification of the electronic conductivity of the oxide film. In both cases, about 10% and 20% of oxygen is reduced to hydrogen peroxide. On passivated surfaces, O2 reduction occurs simultaneously with the reduction of ferric oxide.