The anodic oxidation at high faradaic efficiency of sputter-deposited Al-W alloys, containing 0.1 to 5 atom percent (a/o) W, has been examined using transmission electron microscopy and Rutherford backscattering spectroscopy. The oxidation is revealed to proceed in essentially two stages, with initial formation of relatively pure anodic alumina, neglecting minor contamination from the electrolyte, and subsequent formation of tungsten-contaminated alumina. In the initial stage, tungsten accumulates progressively in a layer of alloy, about 1.5 nm thick, just under the essentially pure anodic alumina film. The accumulation attains levels corresponding to average compositions of the enriched layer of 2.5, 25, and 30 a/o W for bulk alloy compositions of 0.1, 1.8, and 5 a/o W, respectively. The transition from accumulation of tungsten in the enriched alloy layer to incorporation of tungsten species into the anodic film is considered to be related to development of tungsten-rich clusters of critical size within the enriched layer. Subsequently, aluminum and tungsten atoms are incorporated into the anodic film in their alloy proportions in the presence of a steady-state enrichment of the alloy at the alloy/film interface. Due to the discrete nature of the incorporation process, tungsten is distributed nonuniformly, on a fine scale, within the region of tungsten-contaminated alumina : The average migration rate of tungsten species in the film is about 0.38 that of Al3+ ions, thus resulting in films consisting of an outer layer of relatively pure anodic alumina and an inner layer of tungsten-contaminated alumina.