The U.S. Bureau of Mines has investigated methods for the hydrometallurgical treatment of copper, lead, zinc, nickel, cobalt, mercury, gold, silver, and other metals from a number of sources. Extraction procedures were demonstrated with simple and complex sulfide concentrates, scrap metals, smelter waste products, and metal oxides. The procedures developed include leaching with cl2- o2, hcl-o2, fecl2-o2, cacl2-o2, and h2so4-cacl2-o2 at 95 deg to 120 deg c and 30 to 50 psig. Air can be used as the o2 source, and the importance of the reator design is stressed. The treatment procedure used depends on the material being leached, and the methods of subsequent metal recovery determine whether cl2, hcl, fecl2, or cacl2 are available for recycle to the reactor. Metal recoveries are typically 95 to 99 pct, and elements rejected to the leaching residue as insoluble constituents are al, as, cr, fe, s, sb, and si. Methods for controlling the solubility of certain metals in the chloride-oxygen leaching system are also discussed. Descriptions of leaching procedures and extraction results are given.