The presence of alloying elements in aluminium alloys has a significant impact on the anodizing behaviour and results in the formation of porous anodic films with different chemical composition and morphology compared with those generated on pure aluminium. In this work, the effect of alloy element enrichment at the alloy/film interface and of cell diameter on the incorporation and distribution of alloying element species in porous anodic films is considered. It is proposed that above a critical cell diameter, D-crit, the critical alloy enrichment sufficient for oxidation of the alloying element and its incorporation into the film can be maintained across the alloy/film interface. Below D-crit, only a subcritical enrichment can be maintained and the alloying element is then incorporated into the film at the cell boundaries. D-crit depends on the concentration of the alloying element in solid solution and on the critical enrichment. The proposed role of D-crit is supported by alloying element distributions from literature data for model Al-Au and Al-W alloys and new results for anodic films on AA 2024-T3 alloy. (C) 2018 Elsevier Ltd. All rights reserved.