Abstract: According to the mass action law and the coexistence theory of slag structure, the calculating models of mass action concentration for CaO-MgO-FeO-Fe₂O₃-SiO₂,CaO-MgO-MnO-FeO-Fe₂O₃-P₂O₅-SiO₂ and CaO-MgO-MnO-FeO-Fe₂O₃-Al₂O₃-P₂O₅-SiO₂ slag melts are formulated and sulphur distribution between the slag melts and liquid iron is treated. It is found that CaO, MnO and FeO promote desulphurization, while MgO is detrimental to desulphurization. In addition, the sulphur distribution coefficients between the slag melts and liquid iron are presented.