Thiosulfate is an alternative lixiviant for the leaching of precious metals. Research and development of thiosulfate leach technology has been prompted by environmental concerns with the use of cyanide and problems associated with the processing of difficult-to-treat ores. The Newmont Mining, Nevada, operation has successfully developed a combined biooxidation-thiosulfate heap-leaching process for carbonaceous/high sulfide ores and a direct thiosulfate heap-leaching process for carbonaceous/low sulfide ores. The chemistry of copper catalytic thiosulfate leach is a complicated system due to the simultaneous presence of ammonia, thiosulfate and Cu(II)-Cu(I) redox couple, especially for a heap leach operation. In this paper, factors affecting both the leach reaction and the stability of the lixiviant are discussed relating to heap-leaching operation from a solution chemistry perspective. (C) 2003 Elsevier B.V. All rights reserved.