The compositions of three types of passive chromate film on an electrodeposited ZnSn alloy containing 15% Sn were studied both at the surface and at depth by X-ray photoelectron spectroscopy (XPS). Samples of dark yellow chromate films on zinc were used for comparison. The elemental and chemical compositions of chromate films formed on the ZnSn alloy were determined. Differences in the compositions on the surface and in the depth of the films were established. The presence of metallic tin and its compounds was also detected. Changes in the film composition were established after treatment in 5% NaCl. The dark yellow film on ZnSn coating showed the lowest permeability in the corrosive medium studied. The results obtained suggest that the film thickness increases in the order: colourless, iridescent yellow, uniformly coloured dark yellow. The differences in the protective action of the chromate films studied are explained on the basis of data on the compositions of the films and the changes in film thickness as well as corrosive treatment.