Abstract: The sulphide capacity of CaO-SiO₂-Al₂O₃-MgO-FetO slags was studied at 1773 K using gas-slag equilibrium techniques. Utilizing a Pt crucible, the slag was equilibrated with a mixture of gases, namely, CO, CO₂, SO₂ and N₂ to provide the partial pressure of oxygen and sulphur. It was shown that at fixed FetO and Al₂O₃ contents and a fixed (%CaO)+(%MgO)/(%SiO₂) ratio, the sulphide capacity decreases with increasing MgO content. At a constant (%CaO)/(%SiO₂) ratio and constant MgO and Al₂O₃ contents, increasing the FetO content of the slags also results in an increase of the sulphide capacity. The rising basicity of (%CaO)/(%SiO₂) from 1.0 to 1.4 at fixed MgO, FetO and Al₂O₃ contents significantly increases the sulphide capacity.