Abstract The metal sulfide-sulfur dioxide reaction has been characterized for seven different sulfide ores by using the changes in sulfur dioxide pressure during the reaction and by ultraviolet spectrophotometric analysis of the reaction products. The UV analysis, which showed that thiosulfuric acid is a major product of the metal sulfide-sulfur dioxide reaction, was used to monitor the r rate of formation of thiosulfuric acid during the reaction. Results from a series of base-line experiments using ferrous sulfide showed a high correlation between the rate of change of sulfur dioxide pressure, the rate of appearance of ferrous ion, and the rate of formation of thiosulfuric acid. These results were compared with those obtained in similar experiments with six naturally-occurring sulfide ores. Finally, the UV method was employed to monitor the kinetics of the decomposition of thiosulfuric acid into elemental sulfur. Using these results, in conjunction with the mechanisms previously developed for the Wackenroder reaction and the acid decomposition of thiosulfate, a mechanism is proposed for the acid oxidative dissolution of sulfide ores which accounts for the formation of elemental sulfur.