Thermally grown oxide films on carbon steel and titanium were investigated with electrochemical impedance spectroscopy (EIS). EIS data were analyzed in two different ways: by the analysis of imaginary part of the impedance in the high frequency domain and by the electrical equivalent circuit fitting. Three approaches for estimating effective capacitances from constant phase element have been used in this work: Brug, Hsu-Mansfeld formulas and power-law model. The approaches yield different semiconducting characteristics as well as different oxide layer thicknesses. (C) 2013 Elsevier Ltd. All rights reserved.