Molybdate species added to sulphuric acid influence the composition and morphology of the anodic layers formed on 2024 aluminium alloy. In acid media, molybdate ions form polyanions with Mo (+VI). During anodising, different molybdenum oxides are produced, depending on the molybdate concentration. For low molybdate concentrations, unstable molybdenum oxides with Mo between (+IV) and (+V) are principally formed. For high concentrations, these unstable species are also produced but the stable molybdenum oxide with Mo close to (+V) is principally formed. These different electrolyte elements incorporate at the electrolyte/oxide interface and inhibit the transfer of aluminium cations to the electrolyte. This incorporation depends on the molybdate concentration and increases when more stable molybdenum oxides are formed (i.e., at high concentrations). This induces a reduction of the dissolution at the electrolyte/oxide interface. The rate of porous layer growth increases and the diameter of pores decreases. In addition, as the Mo incorporation increases with the increase of molybdate concentration, the influence on porous layer morphology becomes more pronounced. (c) 2004 Elsevier B.V. All rights reserved.