Transmission electron microscopy of ultramicrotomed sections and Rutherford backscattering spectroscopy have been used to quantify the significantly faster mobility of copper ions compared with Al3+ ions in anodic alumina films. To determine the migration rate of copper ions, an Al-0.4 at % Cu alloy film of ca. 35 nm thickness has been sputter deposited onto an electropolished superpure aluminium substrate; the specimen comprising the alloy layer superimposed on aluminium was then anodized at a constant current density to various voltages at high current efficiency. The anodic oxidation of the alloy film results in the prior oxidation of aluminium and the accumulation of copper in a layer of alloy, ca. 2 nm thick, just beneath the anodic film; consequently, no copper is incorporated into the alumina film during anodizing of the alloy. At ca. 49 V for the particular alloy thickness, the alloy film is totally consumed by anodizing and the copper-enriched layer beneath the anodic film is incorporated abruptly into the film, because of the presence of an air-modified electropolishing film sandwiched between the alloy and aluminium regions. With further anodizing, the incorporated copper ions migrate outwards at a rate ca. 3.2 times that of Al3+ ions. The method employed provides a novel approach to determining the precise mobility of foreign ions incorporated into anodic alumina.