The anodic oxidation of Al-4 atom percent (a/o) Cr [7.4 weight percent (w/o) Cr] and Al-10 a/o Cr (17.6 w/o Cr) sputtered-deposited alloy films was investigated by transmission electronmicroscopy and by x-ray absorption studies. The oxidation behavior of both alloys is explained by initial formation of a chromium-free anodic alumina film with accumulation of chromium in a thin underlying alloy layer. On sufficient enrichment of the alloy, chromium and aluminum are oxidized at the alloy/film interface in their approximate alloy proportions. The resultant Cr3+ ions migrate outward through the film more slowly than Al3+ ions. Oxidation of chromium is followed by production of what is probably oxygen gas within the film, which is likely to generate a high pressure, leading to film rupture and gas release. Consequently, a complex, irregular film morphology, associated with voids, channels and cracks, develops. X-ray absorption studies reveal that Cr(VI) species are detected in significant quantities only after the mechanical breakdown of the film. Such species probably form as a result of the high electrochemical potential of the alloy that is freshly exposed to the environment following film rupture.