Abstract: Manganese equilibrium between FeO-MnO-MgO-SiO₂ slags and molten iron has been analyzed by use of the coexistence theory of slag structure. It is shown that $K{'_{{\rm{Mn}}}} = \frac{{{N_{{\rm{MnO}}}}}}{{{N_{{\rm{FeO}}}}[\% Mn]}}$ does not change with basicities, and maintains constant in considerable extent. Calculated [%Mn] by use of the above regularity agree with the measured values. This in turn shows that the deduced calculating model of mass action concentrations for FeO-MnO-MgO-SiO₂ slags reflects the characteristics of this slag system.