The presence of In in aluminium promotes its activation, making it suitable for use as a sacrificial anode. The participation of Cl- ions in the activation effect that In exerts on Al is still a matter of discussion. Therefore, the electrochemical behaviour of In in aqueous and aprotic media containing different anions was studied. The potentiodynamic technique was used in the potential region where it is known activation of Al-In alloys occurs. Metal oxidation in the neighbourhood of the pzc in the presence of Cl-, SO42- and ClO4- ions was observed. The anodic charges involved in aprotic media were greater than in aqueous media where hydrogen evolution reaction (HER) and a metal interaction with OH- ions can take place. These results were explained in terms of specific anion adsorption inducing oxidation of surface metal atoms and leading to the formation of a salt film through which anions migrate. Aspects of salt film formation were analysed using voltammetric equations for semiinfinite diffusion conditions. Diffusion coefficient values of an order of 10(-11) cm(2) s(-1) were obtained, their variation with the potential sweep rate being interpreted as a film aging process. The findings presented in this work demonstrate that the effect exerted by In in aluminium alloys is related to the specific Cl- ions adsorption, at a potential more electronegative than that at which Al activates, avoiding its repassivation.