The growth of anodic films on gallium phosphide has been investigated by transmission electron microscopy using ultramicrotomed sections of anodized specimens and Rutherford backscattering spectroscopy. The films were grown at constant current density, either 450 or 850 mu A cm(-2), in aqueous 0.1 M sodium tungstate electrolyte at 293 K. Two-layered amorphous films, consisting of an outer layer composed of Ga2O3 and an inner layer composed of units of Ga2O3 and P2O5 in the ratio of about 1:3.0, are formed by outward migration of cation species and inward migration of O2- ions. For film formation at 100% efficiency the outer layer represents about 34% of the film thickness. The layering is developed due to the faster migration outward of Ga3+ ions compared with that of P5+ ions. The films are highly soluble in the tungstate electrolyte at the termination of anodizing. However, following the initial period of film nucleation, the films are formed at relatively high efficiency, probably due to the presence or a protective gel layer, composed of hydrated WO3 at the film/electrolyte interface.