This paper presents a study of the copper-ammonia-thiosulfate system for the leaching of gold. The major problem with the thiosulfate system is that for solutions containing copper(II), ammonia and thiosulfate, the chemistry is complex and continually changes due to the homogeneous reaction between copper(II) and thiosulfate. The present paper highlights the effect the changing solution chemistry has on the gold leaching reaction. It will be shown that the gold leach rate decrease as the reaction between copper(II) and thiosulfate proceeds. Although the copper-thiosulfate reaction can be affected by the addition of species such as phosphate, little improvement to the leach kinetics were observed. Thus, the use of oxygen to reoxidise copper(I) to copper(II) was studied, and it will be shown that the oxygen addition rate is critical to leaching, as too much oxygen results in very slow leaching kinetics. Experiments conducted using gold powder and an oxide gold ore showed a similar trend. (C) 2003 Elsevier B.V. All rights reserved.