The Bureau of Mines determined the effect of slag additives on copper solubility and the distribution of typical impurity elements between copper-iron matte and silica-saturated fayalite (iron silicate) slag at 1,250 deg c, in co-co2 atmospheres with 10 pct so2. Small additions (< 10 pct) of cao, al2o3, and mgo were shown to decrease copper solubility of the slag. Additions of cao increased the solubility of bismuth and tellurium in the slag at lower matte grades. Al2o3 additions decreased the solubility of tellurium in the slag, but increased that of bismuth at lower matte grades. Mgo additions increased the solubility of both at lower matte grades. With no additives, matte grade did not affect the solubility of bismuth in slags, whereas increasing matte grade resulted in a slight decrease in the tellurium solubility in slag. Additions of cao or as2o3 alone to the slag increased selenium and arsenic distribution to the matte, but had little or no effect on antimony distribution; whereas small mgo additions decreased antimony distribution to the matte but had no effect on arsenic and selenium distribution. In slags with no additives, selenium distribution to the matte increased with increasing matte grade, while that of antimony decreased and that of arsenic was unchanged.