This paper examines the mechanism of pore formation in anodic films on aluminium. For this purpose, the dimensional changes of specimens during growth of porous films on aluminium in phosphoric and sulphuric acid electrolytes are examined using transmission and scanning electron microscopies. Further, the compositions of films and the efficiencies of anodizing are determined by Rutherford backscattering spectroscopy and nuclear reaction analysis. Significantly, the efficiency of anodizing is about 60%, while the surface of the anodic film is located above the original aluminium surface, i.e. before anodizing. The ratio of the thickness of the anodic film relative to the thickness of the consumed aluminium is about 1.35 for the selected conditions of anodizing. The behaviour runs counter to the widely accepted mechanism of pore formation by field-assisted dissolution of alumina. It is explained by the high plasticity of the anodic alumina in the barrier region in the presence of ionic transport, with film growth stresses displacing material from the barrier layer towards the cell wall region during anodizing. The response of the film to volume constraints on growth indicates a major role of stress and stress-relief processes in determining the morphology and self-regulating organization of pores.