The effect of the passivation potential on the properties of the barrier films formed on high purity aluminium has been examined. The current-time responses were recorded, the charge used during formation of the barrier film was determined, and the steady-state currents were discussed. Impedance measurements provided an insight into the characteristic sizes of barrier films. Equivalent circuits which illustrate the examined aluminium/oxide film/electrolyte systems were proposed and individual circuit elements were defined. The resistance and thickness of the barrier film were shown to increase linearly with increase in the passivation potential, while increase in solution pH caused a marked decrease in the total system impedance. The paper also defines current efficiency in the formation of the barrier film. The value of similar to 20% indicates that the formation of barrier films on aluminium is a process that takes place with considerable current losses. The mechanism of barrier film growth has been examined within the framework of two models suggested by different authors. Both agreements with and differences from the theories proposed have been indicated.