||A study of the rate of dissolution of chalcopyrite (CuFeS2) in acidic solution under oxygen overpressure was carried out by measuring the rate of formation of cupriceions. Effects of temperature, oxygen partial pressure, surface area and concentration of sulfuric acid were evaluated. The temperature was the most sensitive factor among the variables affecting the rate of dissolution. A sized batch of chalcopyrite was leached in the temperature range of 125 to 175oC and in the pressure range of 75 to 400 psia oxygen by using a high temperature - high pressure autoclave. To avoid the contact of the acidic solution, all exposed parts of the autoclave body were completely covered with teflon sheet or tubing and a pyrex-glass liner was employed. In 0.5 N H2SO4 under oxygen overpressures all products of reaction went into solution except for trace amounts of elemental sulfur. No dissolution of chalcopyrite was observed in 1 N H2S)4 at 1750C in the absence of oxygen. The dissolution of chalcopyrite followed linear kinetics and was independent of hydrogen ion concentration. The oxygen dependence indicated adsorption with the rate approaching limiting or plateau values with increasing oxygen dependence indicated adsorption with the rate approaching limiting or plateau values with increasing oxygen pressure. The linear mechanism was explained in terms of steady-state adsorption of oxygen at the chalcopyrite surface followed by the surface reaction. The activation enthalpy for adsorption of oxygen was found to be 41.7 kcal/mole. An activation enthalpy of 7.3 kcal/mole was observed for the surface reaction. Effects of adding FeS04 and CuS04 separately to the initial solution did not accelerate the rate of oxidation significantly. In the case of adding 0.1 M K2S04 to 0.5 N H2S04 solution, the rate was remarkably decreased and yellow jarosite [K2Fe6(OH)12(S04)4] was found to be precipitated. Although the dissolution of chalcopyrite is undoubtedly electro-chemical, the rate data did not reflect voltage dependence. It was concluded that chemical reactions rather than charge transfer processes are rate controlling under the conditions of this study.