Copper-catalyzed thiosulfate leaching is being investigated by the U.S. Bureau of Mines as a potentially economical and environmentally safe method for heap leaching or in situ leaching of low-grade oxidized gold ores. Six variables were investigated in fractional factorial screening tests, including thiosulfate (s2o2-3), sulfite (so2-3), copper (cu2+), and ammonia (as nh4oh) concentration; air- inert atmosphere; and leaching time. Thiosulfate and sulfite concentrations and leaching time influenced gold extraction, while copper affected thiosulfate consumption. Ammonia concentration and leach atmosphere had no significant effect within the experimental region tested. The data from the fractional factorial screening tests were used in face-centered cubic (fcc) surface response experiments to determine the optimum leach conditions for high gold extraction and low thiosulfate consumption. Two models were generated from the data for predicting gold extraction and low thiosulfate consumption within the experimental region. The models predicted 90-pct gold extraction and a consumption of 0.2 Kg s2o2- 3/t ore at 0.20M s2o2-3, 0.00625M so2-3, 0.001.M cu2+, 0.09M nh4oh, and leaching for 48 h. Actual tests resulted in 83-pct gold extraction with 0.4 Kg s2o2-3 consumed /t ore. These results compare favorably to 86-pct gold extraction and 0.21 Kg cn- consumption /t ore using standard cyanidation methods and 24 h leaching time.