Aluminium surfaces are oxidised to form Al2O3 in a freshly prepared anodising bath and in a bath deliberately contaminated with Cl- ions at concentrations ranging from 35 mu g ml-1 to 200 mu g ml-1. It is observed that the presence of Cl- ions in the anodising bath inhibits the oxidation process and the degree of inhibition is dependent on the Cl- ion concentration. It is further observed that a constant electric field of the order of 107 V cm -1 exists across the growing oxide throughout the oxidation process in both unchlorinated and chlorinated electrolytes. Neutron activation, Auger and ESCA analyses of the oxides show that the Cl- ions have diffused into the oxide layer due to the existence of the accelerating electric field across the oxide. It is inferred that the oxidation inhibition in the presence of Cl- ions in the bath is caused by screening of the required oxygen ions by the presence of the Cl- ions in the oxide.