Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Hydrogen cyanide gas can be evolved directly from the pulp surface or purged from the leach tank in oxygen or air that is sparged into the tanks to provide oxygen for the dissolution of gold. Savings in lime consumption can be achieved by leaching at a reduced pH; especially in aqueous solutions high in magnesium such as to be found in the Kalgoorlie Goldfields in Western Australia but a greater percentage of cyanide is then present as aqueous hydrogen cyanide. Another proposed procedure to reduce lime consumption is closed tank leaching where a reduced pH or the natural pH of the ore is wed and hydrogen cyanide loss is prevented by the closed vessel. The dissolution rate of pure gold as a function of pH for constant total cyanide concentration has been determined. Dissolution was directly proportional to the concentration of the cyanide ion present with the contribution to dissolution from aqueous hydrogen cyanide too low to be measured. Ore leached in high magnesium, hyper-saline, process water showed increased gold dissolution at low pH compared to the pure gold. Reducing pH caused move cyanide to be consumed by the pulp but did not significantly increase the amount lost to the atmosphere.