Abstract: On the basis of the coexistence theory of slag structure, mass action concentration for oxides saturated in slag melts are studied.It has been demonstrated that there is a good distribution relation between mass action concentration and activity.Considering this distribution relation, the calculated mass action concentrations for oxides saturated in slag melts agree with the measured activity well.Furthermore CaO-SiO₂,MnO-SiO₂,and MgO-SiO₂,slag melts are discussed and the lgL~l/T,△G°(seperation free energy)~T relations are also deduced.