The electrochemical behaviour of anodically passivated aluminium and aluminium alloys (Al-Cu, Al-6061 and Al-7075) has been investigated in nitric acid and nitric acid containing chloride solutions. The effect of chloride ion concentration on the stability of the anodic oxide film has been studied. Electrochemical impedance spectroscopy (EIS) and polarization techniques are employed. The experimental impedance data have been fitted to the impedance characteristics of a parallel capacitor/resistor combination. The anodic oxide film formed on Al or Al-6061 approaches perfect dielectric behaviour, whereas that formed on Al-7075 or AI-Cu deviate from ideal capacitor behaviour. Anodized Al-6061 electrodes are found to be the most stable either in nitric acid or nitric acid containing chloride solutions. The anodic oxide film consists of two layers-a porous, less stable outer layer and a thin, more compact stable inner one.