Abstract: According to the coexistence theory of slag structure as well as the measured viscosity of Cao-Mgo -MnO-FeO-CaF₂ -Al₂O₃-SiO₂ slag melts at different temperatures and compositions, calculating medels of the mass action concentrations as well as of the viscosity for these melts have been formulated. Calculating results agree with practice, showing that the model of mass action concentrations reflects the structural reality of the melts and tha of viscosity correctiy illustraes its relationship with temperatures and the mass action concentrations of the structural units of the melts.