Electrochemical impedance spectroscopy (EIS) is a common technique used in the assessment of the thickness of anodic oxide films formed on Al and its alloys. However, it is shown here that the resistance (R) values obtained from EIS data are highly dependent on the solution in which the measurements are made, while the capacitance (C) values are independent of the solution conditions and are therefore a much more reliable means of determining oxide film thickness and other properties. We also show that the experimental EIS data for unsealed porous anodic oxides formed on top of a barrier Al oxide film in acidic solutions exhibit only a single time constant, with very little influence seen from the overlying porous oxide layer on the apparent R and C values. The reason for this is demonstrated by utilizing the Laplace equation to calculate the electric field induced in the oxide, which confirms that the EIS response of these films can generally be assumed to reflect the properties and coverage of the underlying barrier oxide film. (c) 2006 The Electrochemical Society.