The Bureau of Mines conducted laboratory-scale weathering research on a leach residue containing elemental sulfur and mixed metal sulfides. Experiments were performed to determine ph and impurity element concentrations in the effluents as functions of time and to determine the effects on effluent concentrations of mixing the residue with dolomite tailing material. Four 800-day tests were conducted simultaneously. Soluble metal concentrations, especially zinc and copper, were excessive in the effluents from washed and unwashed samples of the residue. Concentrations of up to 20 g/l of zn and 12 g/l of cu were obtained. The ph values of the unwashed and washed residue effluents at the completion of the experiments were 2.8 and 1.9, Respectively. Sulfuric acid from oxidation of pyrite caused the ph decrease. Mixing unwashed residue with dolomite tailing to maintain a near-neutral ph minimized the heavy metals content in the effluents. The ph in the dolomite tests increased slowly after the first 360 days and attained values of 7.8 and 8.2, Respectively, for tailing containing 1 pct leach residue and for tailing devoid of leach residue. Dissolved zinc and copper did not exceed 0.10 G/l of zn and 0.001 G/l of cu.