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'_\rmMn = \fracN_\rmMnON_\rmFeO\% 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.