A laboratory method was developed to evaluate the effect of oxygen on the cyanidation of non-refractory gold ores. Eight North American ores were tested. Measurements during the pre-aeration step included oxygen consumption and efficiency, and rate of oxidation as a function of the initial dissolved oxygen. The cyanidation tests determined the effect of oxygen on leaching rate, total gold dissolution and cyanide consumption. The oxygen consumption for the superficial oxidation of the particles depends on the type and concentration of sulphide species in the ore, ranging from 5 to 150 g O2/h/t. Based on these results, plant measurements indicate that the oxidation efficiency of oxygen and air are 8.6% and 0.6%, respectively. For unoxidized sulphides, the rate of oxidation increases markedly with the level of dissolved oxygen. Compared to air injection, oxygen in the leaching step increases the rate of gold dissolution, in some cases enhances the final gold recovery and is often accompanied by savings in cyanide, especially when oxygen is used during pre-aeration. © 1992.