X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface reactivity of polycrystalline Al films in contact with a gas mixture of carbon dioxide and oxygen at room temperature. Based on the characterization of interactions between these substrates and the individual gases at selected exposures, various surface functionalities were identified. Simultaneously dosing both carbon dioxide and oxygen is shown to create surface-terminating carbonate species, which contribute to inhibiting the formation of an Al2O3 layer. Finally, a reaction scheme is suggested to account for the observed dependence of surface group formation on the dosing conditions. (c) 2005 Elsevier B.V. All rights reserved.