The electrochemical behaviour of 99.99% aluminium, an Al-Zn-In and Al-Zn-Hg alloy was investigated, and compared, in aqueous halide, sulphate and perchlorate solutions using electrochemical and surface analytical techniques. Localized attack was observed for pure aluminium and the two alloys (but at more electronegative potential values) in chloride, bromide, iodide and perchlorate solutions but attack of the mercury-containing alloy was observed, also, in sulphate and fluoride solutions. This was attributed to the additional effects of oxide detachment induced by mercury giving rise to large potential oscillations, approaching potential values close to the theoretical potential of aluminium, for immersion in sulphate solutions. The precipitation of oxyfluoride complexes in fluoride solutions leads to the observation of more passive behaviour in fluoride media. The major difference, in the electrochemical behaviour of pure aluminium and the alloys, lay in the extent of metastable attack, as evidenced by the higher frequency of current and potential fluctuations observed with the alloys, before complete breakdown of the passive films occurred.