The anodic oxidation of aluminium alloys is reviewed and discussed with reference to recent results of the authors on the anodizing of model binary aluminium alloys. Attention is given primarily to the oxidation of alloying elements at the alloy/film interface during the formation of barrier-type anodic films. However, the findings are also considered to be applicable to the formation of porous anodic films. The enrichments of alloy layers of about 1–5 nm thickness is revealed to be a relatively common occurrence following anodizing of aluminium alloys. The enrichments, present in the alloy just beneath the anodic film, are a direct consequence of the formation of the anodic film on the alloy. The influence of alloy composition on the enrichments of the alloying element within the alloy is highlighted, and correlated with the Gibbs free energy per equivalent for formation of the alloying element oxide. The development of enriched alloy layers is not confined to anodizing and is found following other surface treatments including chemical polishing, electropolishing and alkaline etching.