The environmental and public concern over the continued use of cyanide in the recovery of gold has grown in recent times due to a number of recently publicised environmental incidents. Of the alternative lixiviants, thiosulfate appears to be the most promising, though the considerable amount of research conducted on thiosulfate leaching of gold over the last three decades has not resulted in its commercial introduction. Perhaps the largest contributing factor to this is the poor understanding of the thiosulfate leach solution chemistry, especially the oxidation of thiosulfate in the presence of copper(II) and oxygen. It has been shown in this research that the oxidation of thiosulfate in the presence of copper(II) and oxygen is very complex with the rates of copper(II) reduction and thiosulfate oxidation being significantly faster in the presence of oxygen. The higher initial rate of copper(II) reduction indicated that oxygen increases the rate of copper(II) reduction to copper(I) by thiosulfate, though the mechanism for this remains unclear. The rates of thiosulfate oxidation and copper(II) reduction were also shown to be affected differently by the presence of anions. This is consistent with thiosulfate oxidation occurring via two mechanisms, with one of these mechanisms involving the oxidation of thiosulfate by copper(II) and the other involving the oxidation of thiosulfate by the intermediate superoxide and hydroxide radicals formed as a result of copper(l) oxidation by oxygen. The effect of various parameters on the rate of thiosulfate oxidation and the copper(II) concentration are also shown. (C) 2003 Elsevier Science Ltd. All rights reserved.