Decomposition of hydrazinium pentafluoroaluminate under oxidative (F-2) conditions leads to aluminium(III) fluoride whose properties are highly dependent on the conditions used for synthesis. In the presence of anhydrous HF, which probably acts as a heat exchange agent, samples have high BET areas, whereas BET areas of samples prepared under gas-solid conditions are small. XPS data are consistent with the presence of Lewis acid centres but, more importantly, emphasise the importance of surface hydroxyl groups, particularly in high surface area compounds. Catalytic behaviour towards isomerisation of 1,1,2-trichlorotrifluoroethane and subsequent dismutations at moderate temperatures and towards room temperature dehydrochlorination of tert-butyl chloride has been demonstrated for some high BET samples: both reactions indicate that surface Lewis sites exist. Catalytic ability is inhibited by surface hydroxyl groups but is improved by prior fluorination of the surface with CCl2F2. Catalysis is inhibited also by the presence of ammonium fluoroaluminate, a by-product of the decomposition process. The [NH4](+) salt level can be reduced by washing with anhydrous HF.