The corrosion behaviour of aluminium in sodium chloride or sodium fluoride solution is at the present time reasonably well understood. In chloride solution, localised attack on the oxide is the most usual problem encountered. In fluoride solution, the Al2O3 film is initially found to be removed to be replaced by a complex oxyfluoride film which subsequently determines the corrosion behaviour. In the present work, potentiodynamic, potentiostatic, open circuit potential and impedance tests are used to elucidate the corrosion behaviour of aluminium in solutions containing both chloride and fluoride ions. It has been found that the presence of fluoride does not lead to localised attack, but that it modifies the surface oxide in such a way as to facilitate attack by chloride. Surprisingly however, for a fixed 0.5 M chloride concentration, these effects of fluoride are only manifest in the concentration range 0.25–10−3 M fluoride at 20°C. Concentrations in excess of 0.25 M have no apparent activating influence. At higher temperatures, lower fluoride levels are sufficient to cause these oxide modifications. The presence of fluoride even in very small quantities prevents ‘film improvement’ reactions similar to those observed in chloride only solutions.