The influence of the addition of small quantities of tin to high-purity aluminium (99.999%) on its electrochemical behaviour at high cathodic potentials was investigated. Cathodic current-time transients were recorded on Al-Sn alloys immersed in neutral 2 m NaCl, when subjected to a potential pulse from a potential around the open-circuit potential to different negative values. Hydration of the oxide layer occurs at potentials more negative than - 1600 mV vs SCE (for alloys with 0.02% and 0.09% Sn), - 1750 mV (Al-0.2% Sn) and - 1900 mV (Al-0.4% Sn), whereupon hydrogen is evolved at the metal-hydrated oxide interface. Anodic current responses to the return of the potential to the rest values were also recorded, and the charges required for oxidation of the substances created during the cathodic processes are discussed.