Electrochemical impedance spectroscopy (EIS) provides a powerful tool for obtaining detailed information on the electrochemical properties of both porous and barrier layers on different aluminium alloys. The impedance value at a given frequency can serve to calculate the electrochemical parameters of the oxide layers represented by each component of the equivalent circuit (EC) which reproduces the behaviour of the studied systems. It is thus possible, with these parameters, to analyse the effects of any factor on the sealing and ageing processes of anodic aluminium oxide layers. Electrochemical results are completed with a detailed analytical study of the oxide layers by XPS, with gravimetric determinations of the changes experienced in the anodising and sealing processes, and with microstructural characterisation of the anodic films by scanning electron microscopy (SEM).