The inhibiting effect of poly-4-vinylpyridine (P4VP) on the corrosion of mild steel and pure iron in hydrochloric acid and perchloric acid was studied. P4VP showed good inhibiting effect, but moleculer weight dependance of the inhibiting effect on the corrosion of mild steel was observed. The addition of chloride ion to perchloric acid affected the inhibiting effect of P4VP. Polarization measurement showed that P4VP suppressed both anodic and cathodic reactions in hydrochloric acid. XPS analysis showed the presence of P4VP and oxide and chloride of iron in the surface layer.