The knowledge of sulfide capacities of FeO-containing slags is necessary for better understanding the resulfurization reaction in hot metal pre-treatments. In this study, copper-iron-sulfur liquid alloy was equilibrated with CaO–SiO2–FeO ternary slag to measure sulfide capacities and FeO activities simultaneously at temperature below the melting point of pure iron. The experimental result that the sulfide capacity increased with increasing FeO content could be explained by the fact that FeO was a basic oxide, and the sulfide capacity of the CaO–SiO2–FeO ternary system was significantly larger than those of the CaO–SiO2–MgO, CaO–SiO2–AlO1.5 and CaO–SiO2–CaF2 ternary systems. Although the FeO activity and oxygen potential were raised by the FeO addition, the calculated value for sulfur distribution ratio between CaO–SiO2–FeO slag and hot metal increased with an increase in FeO content.