Using medium energy ion scattering, combined with Rutherford backscattering spectroscopy, transmission electron microscopy and atomic force microscopy, the development of the copper-enriched alloy layer during anodizing of a sputtering-deposited Al-0.4 at.% Cu alloy has been examined. The enriched layer is located just beneath the amorphous alumina film that is produced on the anodized alloy. Importantly for understanding the mechanism of formation of the enriched layer, the layer is revealed to be of thickness similar to2.1 nm from the start of the anodizing, when enrichment of copper is very low, with no significant increase in the thickness of the layer as the copper enriches to similar to3 x 10(15) Cu atoms cm(-2), the maximum measured in the present experiments. The findings are consistent with a model of the layer in which the copper is present mainly in copper-rich clusters. (C) 2002 Elsevier Science B.V. All rights reserved.