Three sulphidic gold ores containing various quantities of pyrite were tested with the Canadian Liquid Air oxygen uptake measurement method. Oxygen demand of each gold ore is dependent on the pyrite content and its mineralogical origin. The oxidation of sulphide ion into sulphate and iron into ferric ion during pretreatment is beneficial to subsequent cyanide consumption. Gold leaching optimization studies were performed with one gold ore sample. Cyanide consumption was found to be strongly affected by the leaching pH and the "free" cyanide concentration in the slurry. High dissolved oxygen concentration as well as high lead nitrate addition, pH and "free" cyanide concentration favour fast gold dissolution kinetics. The addition of lead nitrate significantly alters the gold dissolution rate behaviour.