A technique was developed for measuring the thiosulfate concentration in gold thiosulfate leaching solutions containing copper and ammonia. The method, which is based on the relationship between thiosulfate concentration and the oxidation rate of silver, was firstly studied using the 'rotating electrochemical quartz crystal microbalance' (REQCM). It was found that the measured silver oxidation rate is directly related to the thiosulfate concentration. The silver oxidation rate was also shown to be unaffected by the presence of other species likely to be found in gold leach solutions. The technique was then evaluated for use as a method of flow injection analysis, utilizing a flow through 'electrochemical quartz crystal microbalance' (EQCM) cell.