Electrochemical impedance spectroscopy (EIS) studies were performed to analyze the passive properties of tantalum and niobium oxides films potentiostatically formed in a 0.1 M KOH solution. The quantitative characterization of these passive materials was carried out through a transfer function previously developed by our research group, which is based on the point defect model (PDM) framework considering the formation of molecular hydrogen. According to the PDM prediction criteria, Ta(2)O(5) and Nb(2)O(5) films exhibited an inherent n-type semiconductor behavior, which was confirmed by the parameters obtained from the fit to the transfer function. The diffusion coefficients of the oxygen vacancies were 0.53 +/- 0.14 x 10(-16) and 2.18 +/- 0.14 x 10(-16) cm(2) s(-1), for Ta(2)O(5) and Nb(2)O(5), respectively. And a slight increase of the corresponding hydroxyl vacancies diffusion (2.73 +/- 0.02 and 2.23 +/- 0.65 x 10(-16) cm(2) s(-1)) was obtained, suggesting the favorable diffusion of these defects due to the alkaline conditions. (c) 2009 Elsevier B.V. All rights reserved.