The electrochemical properties of the system Al-Mo alloy vertical bar anodic oxide film vertical bar 1.0 M HCl solution were investigated as a function of alloy composition and applied potential in 1.0 M HCl. Voltammetry measurements have revealed that the alloy systems with x(Mo) >= 0.50 behave almost like pure Mo. The alloy systems with x(Mo) <= 0.45 showed the potential regions of primary and secondary passivities in which the structural and protective properties of the anodic oxide films were studied using electrochemical impedance spectroscopy. Orazem's graphical methods and Mott-Schottky analysis were used to extract the values of parameters of the constant-phase element, interfacial capacitance, ohmic resistance, type of conductivity, and concentration of charge carriers in the semiconducting oxide film, which are all from the high frequency impedance data. The pseudoinductive behavior observed at low frequencies in the potential range of the secondary passivity was discussed in terms of the surface-charge approach based on the chemistry of the point defect model. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3240875] All rights reserved.