Properties of thin oxide films on Al and Al-Sn alloys (with Sri content of 0.02, 0.09, 0.20 and 0.40 wt.%) formed either naturally or anodically in borate buffer solutions were investigated by means of electrochemical impedance spectroscopy. Equivalent circuits have been proposed that completely illustrate the Al(Al-Sn alloy)/oxide film/electrolyte systems examined, and properties of oxide films were determined. The stability (thickness and resistance) of oxide films has been found to increase with increased Sri content in the alloy, with increased passivation potential, and with longer time of anodising. The increase in temperature of anodising significantly reduces impedance in systems observed.