The average thickness and composition of the copper-enriched alloy layer that is present at the alloy / oxide interface during anodic oxidation of an electropolished Al-0.9 at.% Cu alloy at a constant current density of 50 A m(-2) have been determined by Rutherford backscattering spectroscopy and transmission electron microscopy. The copper-enriched layer, of about 2 nm thickness, has an average composition of Al-40 at.% Cu and contains about 5.4x10(15) Cu atoms cm(-2), The average composition and thickness of the layer do not change significantly during anodizing from 10 to 200 V. The essentially steady-state, copper-enriched layer is established mainly by the prior electropolishing of the alloy. As a consequence of the pre-enrichment of copper, both aluminium and copper atoms are oxidized immediately at the alloy/film interface on subsequent anodizing. Owing to the importance of copper enrichment in AI-Cu alloys to the oxidation of copper atoms, alloy pre-treatment has an important role in determining the initial oxidation behaviour.