The U.S. Bureau of Mines is studying the effect of supercritical water, at 400 deg. C and 3,500 psi, and of subcritical water at lower temperatures and pressures, on mixtures of minerals and metals to determine the possible applications of water at high temperature and pressure for processing these materials. This research was conducted using a flowthrough bench-scale system. Copper-bearing minerals studied were chalcocite, chalcopyrite, covellite, and cuprite. In general, mixtures of copper minerals and pyrite reacted to form bornite. Intermediate phases formed were chalcopyrite and digenite. Similar reactions occurred between pyrite and metallic copper. A mixture of chalcopyrite and pyrite was inert in supercritical water. The reaction between cuprite and metallic zinc yielded metallic copper. The cuprite was reduced by hydrogen generated by the reaction between metallic zinc and water. None of the reactions occurred exclusively in supercritical water, but also occurred in water at high temperature and pressure below the critical point. Experimental results did show that a dual-reaction process, consisting of solid-state diffusion and fluid transport, was occurring.