We demonstrate the ability of a polyaniline film, into which highly dispersed platinum microparticles were introduced at a fairly low level (ca. 25 mu g cm(-2)), to protect stainless steel (containing 15.86% Cr) against corrosion in strong acid solution (2 M H2SO4) We show that the electrocatalytic reduction of oxygen (from air) on the platinized polyaniline film can be coupled to the self-passivation of steel, i.e. to the anodic formation of a passive layer composed of the oxides or oxyhydroxides of iron/chromium. The reduction of O-2 at dispersed platinum microcenters, is apparently fast enough to replenish the polyaniline charge consumed by the oxidation of stainless steel, and to poise the steel＇s potential at about 0.4 V vs SCE. Consequently, the corrosion potential of stainless steel is stabilized within the passive range, and the rate of its dissolution is minimized. Also a low overpotential for the reduction of protons seems to facilitate the active-to-passive transition of stainless steel.