The mechanism of stress generation during the anodic oxidation of pure aluminium in aqueous H2SO4 solutions has been explored using a.c. impedance spectroscopy and a beam deflection technique. From the analysis of a.c. impedance data, the resistance of the anodic oxide film was found to decrease as it grew progressively in 0.5 M H2SO4 solution, indicating an increase in the concentration of aluminium vacancies V-Al(3-)(ox) within the film. From measurements of the deflection, compressive stresses were always observed at relatively low current density of 2 mA cm(-2) during the growth of the film in aqueous H2SO4 solutions. Based upon the experimental results obtained from this work, it is suggested that stresses developed which determine the sign and magnitude of the deflections, are not distributed entirely over the whole oxide film, but are limited to a narrow region of the aluminium/oxide him interface below 1 nm. The changes in the sign and magnitude of stresses were satisfactorily accounted for in terms of the annihilation of V-Al(3-)(ox) and the generation of oxygen vacancies V-O(2+)(ox) at the aluminium/oxide film interface. From this, a new model for stress generation during the anodic oxidation of valve metals has been developed.