This paper discusses Bureau of Mines investigations of the feasibility of partitioning complex sulfit( minerals to simpler sulfide phases by sulfidation reactions. Such partitioning can afford opportunity to improve selective extraction of mineral constituents of value. This report describes research involving sulfidation partitioning of byproduct chalcopyrite concentrates from Missouri lead ore processing, and a Utah chalcopyrite concentrate from cu ore processing. Small chalcopyrite samples were sulfidized with elemental s in glass capsules sealed after evacuating and backfilling with he, by heating for 1 to 24 h at specific temperatures in the range of 250 deg to 500 deg c, extending from below to above the boiling point of s (approx. 445 Deg c). Above the boiling point of s, the overall reaction with adequate s (13.9 Wt pct) is 5cufes2 + 4s0 --> cu5fes6 + 4fes2. The reaction below the boiling point of s, with adequate s (17.4 Wt pct), results in partitioning to the simple cu sulfide, covellite, according to the reaction cufes2 + s1 0 --> cus + fes2. The covellite forms as a crust on particulate pyrite cores. Larger amounts of concentrate were sulfidized to the covellite-pyrite product in a tube furnace reactor.