The Bureau of Mines is investigating chlorine hydrometallurgy techniques for recovering metal values from complex sulfide concentrates and smelter mattes. One of these techniques involves the reaction of chlorine with an aqueous slurry of concentrate, followed by the addition of oxygen at 25 to 50 psig. The leach reaction is highly exothermic, and the reaction temperature is held constant between 110 deg and 115 deg c by cooling. Dissolved metal values are recovered from leach solution by several unit operations, such as solvent extraction, brine leaching, crystallization, and fused-salt electrolysis. The chlorine-oxygen leaching technique was shown to be effective for extracting metal values from complex lead- zinc-copper concentrates and from a copper-lead matte containing cobalt and nickel. Metal recoveries of 95 to 99 pct were obtained for lead, zinc, copper, cobalt, silver, and nickel. A similar technique, employing a ferrous chloride-oxygen leaching procedure, was employed to extract metal values from concentrates that contain as much as 45 pct arsenic along with copper, cobalt, and silver. Respective silver and cobalt recoveries of 99 and 97 pct were obtained. Both leaching systems reject the arsenic, antimony, sulfur, and iron contaminants to the tails.