Transmission and scanning electron microscopy and electrochemical impedance spectroscopy measurements were made on anodic layers on 1050 and 2024T3 aluminium alloys, prepared from solutions of phosphoric acid, boric acid and sodium tetraborate. The microstructural parameters were dependent on the aluminium substrate and the type of electrolyte. A structural model for the oxide layer was generated from the data in the case of 1050 alloy. We showed that a similar model could not be made for the 2024 alloy, because of the disorganised morphology of the porous layer. The a.c. impedance behaviour of the layers was investigated. According to the characteristics of the equivalent circuit, we showed that electrochemical impedance spectroscopy provided a method for the evaluation of the thickness of the barrier layer, whatever the morphology of the oxide layer: well-organised (1050 alloy), or disorganised (2024T3 alloy). This method gave the same results as those obtained using transmission electron microscopy.