The ellipsometric measurements of the passivity and pitting of electrolytic iron were carried out in borate buffer solution containing sulfate ion (pH 8.45). Iron specimen suffers pitting corrosion at potentials more positive than a critical value in the neutral sulfate solution. At perfect passive potentials, neither the optical constants nor the film thickness of the passive film is affected by the presence of sulfate ion, while the pitting causes an abrupt change in film structure, due to the formation of anodically deposited ferric hydroxide film. Microscopic examinations have revealed the formation of ferrous precipitate in the pit. It is suggested that sulfate ion takes part in the formation of ferrous hydroxide inclusions in the passive film, and that pits are generated by the inclusions which provide preferential paths for the diffusion of ferrous ions.