The passivation process of iron in borate buffer solution (pH 8.4) was investigated by using linear potential sweep voltammetry, current step chronopotentiometry, open circuit chronopotentiometry, and alternative current (ac) impedance spectroscopy. Iron is passivated when the potential is in between 20.45 and 0.9 V (vs. saturated calomel electrode). The capacitance of the passive film on iron in its passive region was obtained by fitting experimental ac impedance spectra with a Randles equivalent circuit. At the potential higher than 0 V in the passive region, the reciprocal capacitance of the passive film was proportional to the potential, which is ascribed to the thickening of the passive film. At the potential lower than 0 V in the passive region, there was a proximately linear relationship between the reciprocal square capacitance and the potential as well as between the reciprocal capacitance and the potential. This is because the capacitance value at this potential region equals almost unity in F m(-2). Only at a narrow potential region (-0.3-0 V) can the linear relationship of reciprocal square capacitance with potential be ascribed to the Mott-Schottky behavior, because there is no change in the thickness of the passive film at this potential region. The donor concentration of the passive film determined by the Mott-Schottky relationship was 1.13 x 10(27) m(-3) at -0.3 V. The dielectric constants of the passive film varied from the potential, and were in between the dielectric constants of Fe3O4 (250) and gamma-Fe2O3. (C) 2004 The Electrochemical Society.