The growth of anodic alumina in sulphuric acid is investigated at constant current on bulk and sputtering-deposited aluminium. The ratio of the thickness of the film to the thickness of oxidized aluminium is shown to increase with increase of the current density (from 0.5 to 50 mA cm −2 ) and with decrease of the electrolyte temperature (from 20 to 0°C). In addition, the sulphur content of the films and the efficiency of film formation increase. It is suggested that pores are generated primarily by dissolution at current densities below ∼2 mA cm −2 , with flow of film material dominating at higher current densities.