The composition and morphology of the double-layered anodic film formed at 5 mA cm(-2) on InP in aqueous sodium tungstate electrolyte is examined by transmission electron microscopy (TEM), atomic force microscopy (AFM), Rutherford backscattering spectroscopy and nuclear reaction analysis. The outer layer is composed mainly of In2O3 of relatively low atomic density, about 50% that of crystalline In2O3. The low density is probably due partly to the presence of numerous cavities in the layer, revealed by TEM and AFM. The inner layer is enriched in phosphorus relative to the composition of the substrate, with a P : In atomic ratio of about 2.17. From the measured ratio of phosphorus to indium, the composition of the inner layer can be expressed as either In2O3.2.17P(2)O(5) or In(PO3)(2.17). The average nm V-1 ratio is 1.99 +/- 0.07. However, there are wide variations of the local film thickness, associated with roughness at the substrate/film and film/electrolyte interfaces and local variability in the ionic current density due to cavities in the film. The film forms with a small loss of indium species to the electrolyte. The outer layer represents about 32% of the total thickness of a film formed at 100% efficiency.