An electrochemical study of the effects of gallium, as an alloying component and present as ions in solution, on the passive behaviour of 99.999% aluminium is reported. It was possible to deposit gallium onto the aluminium surface by a cathodic polarization at -2.0 V(SCE) in halide solutions containing gallium ions. This was achieved by an alkalization process which favoured the formation of hydrolysed gallium species of the form in [HGaO3]2- and [GaO2]- which subsequently led to the formation of metallic gallium at the surface. Al-Ga alloys with low levels of gallium displayed, also, more active behaviour following cathodic polarization. This enrichment of gallium at the surface had an activating effect on the aluminium substrate. Complete activation could be achieved only if a certain amount of gallium was present as a solid solution. It is proposed that the role of gallium is to bias the surface charge in the direction of activation, and that activation occurs only when sufficient bias has been achieved. This will depend on the surface concentration of gallium and the concentration of gallium in solid solution. Once this condition is satisfied halide adsorption occurs more readily at the biased surface and attack proceeds through the formation of a number of aluminium complex species.