Thiosulfate is a potential alternative to cyanide in the recovery of gold. However, in the presence of copper and ammonia the solution chemistry is complex, and the thiosulfate concentration continually decreases due to its oxidation. The determination of the thiosulfate concentration by traditional techniques is difficult in leach Solutions due to the interference from copper and other polythionates. A flow injection analysis method for thiosulfate has been developed which utilizes an electrochemical cell containing platinum and silver electrodes. The basis of the analysis is that the silver electrode measures the formation of the silver thiosulfate complex plus the oxidation/reduction of copper(I)/copper(II). In order to correct for copper, the charge for copper(I)/copper(II) is measured on the platinum electrode, and this is subtracted from the silver charge measured on the silver electrode. This provides a simple and robust analytical method for the analysis of thiosulfate in solutions used for gold leaching. (C) 2004 The Electrochemical Society.