Measurements are reported for the variation of the open-circuit potential, Er, of aluminium in oxygen-saturated sodium salt solutions. The value of Er was independent of SO42- and NO3- concentrations and similar to the value obtained for water (0.04 (s.h.e.)). Er was a function of chloride concentration given by                     Er = -0.475-0.060log[Cl-] V (s.h.e.) at 25°C. There was a less well defined relationship between Er and NO2-, I- and Br-, and a complex relationship with F-.     The potentiodynamic characteristics are reported for aluminium in 1-0.01 mol l-1 Cl- oxygen-saturated solutions. Functional relationships were found for E0, Ep, Es and E0' with chloride activity at 5, 25, 50 and 75°C. Hysteresis effects are reported.     The experimental results are interpreted in terms of a thermodynamic equilibrium condition between the surface oxide and soluble aluminium chloride. As the system oscillates across the equilibrium conditions the surface will passivate or pit. A critical bulk solution chloride concentration is necessary to maintain the growth of the pit; the experimental value was 1.6 mol l-1 Cl- and the corresponding open-circuit potential was Ecrit = -0.48 V (s.h.e.). The pitting potential, Ev, was interpreted as an overpotential, ?p, given by ? = Ep,- Ep-Ecrit.