Electrolytic dissolution of pure aluminium in aqueous halide electrolytes has been investigated and found to exhibit the behaviour conventionally termed the "difference effect". The weight loss from the aluminium anode was greater than predicted by Faraday’s law for trivalency of aluminium, and hydrogen gas was simultaneously evolved from the anode. A quantitative relationship has been established between the extent of these processes and the magnitude of the applied anode current density in the range 10⁻² to 10² amp/cm². This and related information has been used to demonstrate the inadequacy for aluminium of previously advanced explanations such as the "local corrosion" hypothesis and the "monovalent aluminium cation" hypothesis. To account for the observed phenomena, a statistical model of the anode behaviour has been developed, based on the hypothesis that aluminium metal can react non-electrochemically with water. When the experimentally determined time dependent anodic kinetic parameters of Earl (5) and Watson (6) are incorporated, a satisfactory quantitative account of the electrode behaviour is achieved. The relevance of the results to practical corrosion of aluminium, especially pitting, has been established.