The results of impedance spectroscopy measurements confirm that different types of anodic films grow on bismuth in slightly alkaline solution and under potentiodynamic and potentiostatic experimental conditions. The band model of solids can describe the behavior of these films. Several approximations were necessary to introduce, since "rapidly" grown oxide films (with alpha = 22 nm V-1) were highly nonstoichiometric and amorphous in structure. Numerical analysis showed that two capacitive contributions were involved in the measured impedance spectra, the oxide film capacitance (C-ox) and space charge capacitance (C-SC), which were used to characterize the semiconducting and dielectric properties of the Bi2O3/electrolyte structure. Both donor concentration (N-D) and the critical electrode potential have been determined from Mott-Schottky behavior. The dielectric properties of the oxide film were discussed in terms of a parallel plate capacitor and in accordance with the high-field growth law, and several parameters were determined : the oxide layer thickness (d(ox)), the dielectric constant (epsilon), the potential at which oxide electroformation starts (E-ox(d=0)), the thickness of native oxide (d(in)), and the anodization coefficient(alpha). Potentiostatic anodization confirms the rearrangement within the oxide film under the high electric field detected by impedance spectroscopy. The results indicate that the film formed potentiostatically behaves almost like a capacitor (insulator). The high resistance of the Bi-Bi2O3/electrolyte structure was ascribed to a very high interfacial charge-transfer resistance.