Inhibition effects of bismuth(III) chloride on cobalt corrosion in 1N HClO4, 1N H2SO4, and 1N HCl were investigated by polarization measurements and colorimetry of dissolved cobalt ion. More than 85% of the inhibition efficiency was obtained for cobalt in these acid solutions. X-ray photoelectron spectra showed the formation of a protective film which comprises an inner layer composed of metallic bismuth and an outer layer of cobalt(II), bismuth(III), chloride, oxide, and hydroxide ions. Because the cathodic bismuth deposition was stimulated and a uniform layer of bismuth was not formed on the surface, the corrosion reaction was accelerated in 1N HCl at a high concentration of the inhibitor. The inhibition efficiency of the compound for various metals in 1N HClO4 was confirmed as being associated with a difference in the standard electrode potential between bismuth and a metal.