The effects of applied d.c. potential and polarization time on the passivation of stainless steel 304 (SS304) were investigated in deaerated 1 M NaHCO3 aqueous solutions at pH 8. Electrochemical impedance spectroscopy was used in conjunction with a rotating disc electrode. The data were analysed by considering an equivalent circuit. The changes in impedance parameters at applied d.c. potential signal changes in the properties of passive films on SS304 and allow to distinguish the parameters at low potential (-0.6 to 0.3V vs SCE) from a different one at high potential (0.5 to 0.8V vs SCE). The oxidation reactions were controlled by both charge transfer and mass transfer processes. Diffusional resistance was high for both passive films and was considered to represent the resistance to movement of ions or vacancies through the surface layer of oxide films. It is deduced that the passive film present in the low potential region is partially dissolved at 0.4V vs SCE and that a new passive film is formed in the higher potential region. The equivalent circuit used to obtain the best fit and the fitting parameters was dependent on the electrode potential and the polarization time. The reproducibility of the impedance spectra at constant potentials demonstrate that the passive film formation is highly irreversible process. No traces of localized corrosion were detected but, for a high potential and long polarization time, the electrode surface coloration to a uniform gold colour confirms the film thickening.