When the chemical environment near the surface of an iron electrode was changed artificially, potentiostatic current oscillations in the Fe/0.5 mol dm(-3) H2SO4 system can be generated. The effects of adding OH- and SO42- on the periodic behavior of iron in 0.5 mol dm(-3) sulfuric acid were studied by this method. It is suggested that the prepassivating film is a mixed film of FeSO4 and Fe(OH)(2), and the ability of prepassivating film to protect the iron electrode from electrodissolution is decided by the thickness of the mixed prepassivating film and the proportions of FeSO4 and Fe(OH)(2) in the mixed film, which will affect the value of the current density during anodic polarization. The precipitation of Fe(OH)(2) is the prerequisite of the potentiostatic current oscillations during the transition from prepassive to passive regions.