The continued use of cyanide in the recovery of gold is being questioned due to environmental and public concerns. The present paper describes research into the gold thiosulfate leaching system, which is considered to be the most promising alternative to cyanide. The thiosulfate system is complicated by the homogeneous reaction between two of the reactants, copper(II) and thiosulfate. This reaction produces copper(l) and polythionates. It is generally accepted that oxygen addition is thus required in order to regenerate copper(II) from copper(l). Despite this, no one has previously studied the effect of oxygen addition on the kinetics of gold leaching in thiosulfate solutions. This paper presents a study on gold leaching in thiosulfate solutions containing oxygen. It will be shown that the leaching reaction is severely hindered as the reaction between thiosulfate and copper(II) occurs in the presence of oxygen. It is postulated that an intermediate product of the oxidation of thiosulfate by copper(II) in the presence of oxygen is responsible for the slow leaching kinetics. This effect was not observed when oxygen is excluded from the solution. Disulfite (S2O52-) was identified as a possible cause of this effect, as experiments performed with the addition of disulfite also resulted in very low gold oxidation rates. (C) 2003 Elsevier Science Ltd. All rights reserved.