Thiosulphate is a metastable anion that tends to readily undergo chemical decomposition in aqueous solutions. The presence of tetrathionate, trithionate and sulphide is unavoidable in the ammoniacal thiosulphate system, as they are products of thiosulphate decomposition. Thiosulphate decomposition is of great importance in the thiosulphate leaching of gold. The effect of three typical sulphur species tetrathionate, trithionate and sulphide on thiosulphate leaching has been investigated in both pure gold and ore systems. The gold dissolution increased at low concentrations of the sulphur species, but decreased at high concentrations. The presence of these sulphur species significantly reduced thiosulphate decomposition, and this beneficial effect became more prominent at high levels of the species. The Eh-pH diagram for the Au-N-S-H2O system indicated that the sulphide ion could form HS- to complex gold, enhancing gold leaching. Tetrathionate, trithionate and sulphide would convert to thiosulphate with some elemental sulphur being formed via oxidation or reduction routes. The presence of the sulphur species shifted the thiosulphate decomposition reactions, and hence stabilised thiosulphate. Raman spectra demonstrated that the gold surfaces were readily passivated in the presence of the sulphur species, and the passivation tended to be more severe at higher levels of the species. (C) 2006 Elsevier Ltd. All rights reserved.