Tetraethyl lead is a constituent of most fuels of high anti-knock value, but such fuels cause serious corrosion of magnesium-base alloy tanks and may completely penetrate them in a few months. No work on the mechanism of this attack has been published and no satisfactory method for protection exists, so that magnesium-base alloy tanks cannot be used for leaded fuels. Since tanks constructed from an ultra-light alloy would be of advantage for aircraft, experiments have now been carried out to elucidate the mechanism of their corrosion, this work representing the first stage of an attack on the problem of providing protection against such corrosion.