The mechanism of ionic transport in the amorphous anodic film formed on Al–40 at.% Sm alloy has been examined by transmission electron microscopy and Rutherford backscattering spectroscopy. The film consists of an outer layer, about 6% of the total film thickness, composed of relatively pure Sm2O3 and an inner layer containing units of Al2O3 and Sm2O3 distributed homogeneously at the available resolution. The anodic film material is formed by migration of O2−/OH− ions inward and migration of cations outward, with a cation transport number about 0.29. The two-layered film develops as a consequence of faster migration of Sm3+ ions than Al3+ ions in the film.