Infra-red reflectance spectra showed that the protective films formed on Cu by benzotriazole were composed of a highly inert Cu benzotriazolate (CuBTA) polymer. The spectra also indicated that such films continued to grow beyond the chermisorbed monolayer stage to form films up to several thousand Ã… thick. Film growth involved precipitation of Cu ions as highly insoluble CuBTA complexes at the solid/liquid interface. Polarization curves showed that the CuBTA films acted as physical barrier layers. These barriers markedly inhibited both the anodic and cathodic corrosion reactions in neutral, aerated or deaerated solutions. Anodic and cathodic overvoltages increased monotonically with the CuBTA film thickness. In acidic solutions (i.e. pH = 3), the CuBTA films acted primarily as cathodic inhibitors of the h.e.r. but not of the oxygen-reduction reaction.