This study was carried out on a finely disseminated gold-bearing ore from Kaymaz, Turkey. The main objective was to determine the mechanism of reaction between thiourea and Fe 31 in the thiourea leaching of gold. Thiourea consumption levels were also examined during the leaching operations for different leaching parameters, i.e., molar ratio of ferric sulfate/thiourea, pH, and temperature. The leaching using the thiourea process was compared with cyanidation. In optimum leach conditions, 85.8% gold extraction was obtained with feed of -53 mum particle size in 6 h. (C) 2002 Elsevier Science B.V. All rights reserved.