Poly(o-phenylenediamine) (PoPD) has been successfully electropolymerized by cyclic voltammetery on 304 stainless steel from sulfuric acid solution containing o-phenylenediamine monomer. Thin, adherent, and transparent layers of PoPD which have a ladder conductive polymer structure were formed. The layers facilitated the passivation of the stainless steel (SS) in acid solution. The underlying oxides obtained by aging in acid solution are investigated after peeling off the polymer layers and compared with those formed under polyaniline layers and by applying a constant positive potential. The passive film under PoPD exhibits a superior resistance for pitting in aerated 3% NaCl to other examined passive films. It is shown that the passive film under PoPD has a high Cr content, including other constituents of SS. The film has been characterized as an oxyhydroxide structure. The protection role of PoPD for SS is discussed here. (C) 2006 The Electrochemical Society.