The semiconducting properties of passive films formed on AISI type 304 stainless steel in borate buffer solution were studied by capacitance (Mott-Schottky approach) and photocurrent measurements. The oxide films formed on 304 stainless steel in air in the temperature range between 50 and 450 degreesC have also been studied. The results obtained show that, in all cases the electronic structure of the films is comparable to that of a p-n heterojunction in which the space charges developed at the metal-film and film-electrolyte interfaces have also to be considered. This is in accordance with the analytical results showing that the oxide films are in all cases composed of an inner region rich in chromium oxide and an outer region rich in iron oxide. When the temperature of the film formation changes the thickness of the films and the doping densities also change but the model to explain the semiconducting properties of the very thin passive films and the relatively thick well-crystallized thermally grown films is basically the same. To confirm the existence of the heterojunction measurements on thermally grown films before and after the cathodic reduction were also carried out.