Passive layers formed electrochemically at constant potentials are studied by impedance spectroscopy in 1 M KOH. In the high frequency range, they act as a cpe. Passive layers change with potential and at lower frequencies, adsorption or insertion reactions are observed in several potential ranges. Passive layers have over a limited potential range, p-semiconductive properties that depend on their preparation conditions (time, potential) : the number of accepters decreases with the formation time, and the Bat band potential E(fb) decreases when the formation potential increases. Due to the shift of kinetical processes towards high frequencies in some potential ranges, the frequency dispersion in the high frequency range varies with potential, leading to an apparent variation of the E(fb) values deduced from the Mott-Schottky plots.