The effects of additions of fluorozirconic acid to sulphuric acid on the anodizing behaviour of aluminium have been investigated under a constant voltage at temperatures of 0 and 20 °C. The fluoroacid increased the rate of film growth, with a dependence on the fluoroacid concentration, the electrolyte temperature and the anodizing time. Compositional analyses showed that fluorine species were present in the films. However, zirconium species were absent. The fluoroacid generally enhanced film dissolution, although this effect was less important at low fluoroacid concentration, low electrolyte temperature and short anodizing times. Sodium fluoride additions influenced the anodizing behaviour similarly to the fluoroacid.