Abstract: In accordance with the coexistence theory of slag structure, CaO-SiO₂ and CaO-FeO-SiO₂ phase diagrams and corresponding thermodynamic data, a calculating model of mass action concentrations for CaO-FeO-SiO₂ melts at 1 600℃ has been deduced. The calculated contours of iso-mass action concentration N_FeO are fairly identical with the contours of iso-activity α_FeO measured. The apices of iso-N_FeO contours are located at the range of slag basicity 1.703 to 1,778, so for the decarburization and precipitation deoxidation the slag basicity had better be kept in that range, and on the contrary for diffusion deoxi-dation the slag basicity had better be kept off that range. Similarity of iso-silicates mass action concentration contours to the corresponding iso-thermal contours of CaO-FeO-SiO₂ phase diagram shows that silicates play an important role in the melting temperatures of slag system mentioned.