Anodic oxide films grown on Si in an aqueous NH4OH solution have been characterized by in situ. ac impedance techniques and their behavior compared to thermal SiO2 oxides. Mott-Schottky analysis of oxide-free p- and n-type Si leads to doping concentrations which are in excellent agreement with independent data of conductivity measurements. The growth of anodic oxides can be monitored with in situ impedance measurements. Anodic oxide films exhibit a nonideal capacitive behavior and hence a constant-phase element has to be introduced for the evaluation of the experimental results in terms of an equivalent circuit. For oxides thicker than approximate to 50 Angstrom the roughness factor for the oxide surface can be determined by a comparison of the capacitance results with the film thickness obtained from x-ray photoelectron spectroscopy (XPS) measurements. For thinner oxides the space-charge capacitance in the Si and the effect of the surface roughness have to be considered to obtain a good agreement of impedance and XPS data. A comparison of impedance spectra of anodic and thermal oxides shows that anodic oxides not only exhibit less ideal capacitive behavior but also show a drastically lower charge-transfer resistance indicating a relatively high mobility of ions in the anodic oxide film. The nonideality of the anodic oxides is also evident from cyclic capacitance-voltage (C-V) measurements in which a strong hysteresis is observed in contrast to thermal oxides for the anodic oxide films. Furthermore, the dielectric constant of the anodic oxides is higher than for ideal SiO2 films, which can most probably be attributed to the presence of hydroxides in the film. The quality of the anodic oxide films can be significantly improved by annealing. The imperfect nature of the as-grown anodic oxide films is also reflected in a lower chemical resistance to etching in NH4OH solutions compared to thermal oxides. By postoxidation annealing also the chemical resistance of anodic oxides is greatly improved.