The evolution of the electrical properties of Nb2O5 and WO3 films, at different formation potentials (E-f), in 0.1 M HClO4, were followed by electrochemical impedance spectroscopy (EIS). During the growth of the oxide films, EIS spectra were acquired every hour, maintaining the potentiostatic pulse during 20 h. The Nb2O5 and WO3 impedance spectra show variations vs potentiostatic aging. However, the Mott-Schottky analysis showed that the density of donors (N-D) does not depend on the potentiostatic aging but depends on the E-f, while, the flatband potential (E-FB) is almost constant with the potentiostatic aging and E-f, for both oxides. The relationship between the invariability of the semiconductor properties (N-D and E-FB) with the potentiostatic aging and the variation of the impedance spectra, in the case of the W, can be explained by the formation of an external layer of WO3 center dot(H2O)(x), which must be very thin; the detachment of this layer causes a very small difference in the thickness, not modifying the semiconductor properties (N-D and E-FB), but greatly changing the electric properties of the film, since WO3 center dot(H2O)(x) is more resistive. A similar hydration process may occur in the Nb2O5 films to a lesser extent.