The corrosion and passive behaviour of aluminium in chloride, bromide and iodide solutions in the absence and presence of indium, as activator ions, was investigated using electrochemical techniques. Pit formation mechanisms for aluminium in chloride-, bromide- and iodide-containing solutions appeared to be similar with the caveat that, in the case of iodide anions, liberated iodine attacked the base metal leading to destabilization of the forming film. In addition, the formation of iodic and hypoiodous acids gave rise to the build-up of a highly aggressive pit solution which inhibited repassivation. The difference in the activity of the halide anions was evident also on activation of the aluminium surface by indium ions introduced into the working solution. Greater adsorption of bromide and iodide anions at flawed areas reduced the rate and efficiency of activator deposition at the aluminium surface, with the result that the rate at which the surface was activated was reduced. Activation appeared, also, to be more permanent in chloride-containing electrolytes.