The sulfide capacities of the CaO–SiO₂–MnO–Al₂O₃–5 mass% MgO slags were measured at 1873 K over a wide composition range using a gas–slag equilibration method. The effects of basicity and the activity coefficient of sulfide on the sulfide capacity of molten slag were also investigated based on the structural view of silicate melts. In the multicomponent silicate melts containing high MnO (up to about 50 mass%), the sulfide capacity mainly increased with increasing MnO content. The capacity and modified Vee ratio, i.e. (CaO+MnO+MgO)/ (SiO₂+Al₂O₃), showed a good linear relationship. Assuming that the basicity and the stability of sulfide ions in the slag are proportional to the activity of basic oxides and the activity coefficient of sulfides, the composition dependency of the sulfide capacity is well described by changes in the a_MO to γ_MS (M=Ca, Mn) ratio. The substitution of silica by alumina did not affect the sulfide capacity of the slags not only because of an increase in the activity of basic oxides but also because of a decrease in the stability of sulfides as Al₂O₃/SiO₂ ratio increased. In the high silica melts of which silica content greater than about 30 mass%, the sulfide capacity increased with increasing MnO/CaO ratio, whereas it decreased by increasing the MnO/CaO ratio in the low silica melts (< about 30 mass%). This tendency of sulfide capacity resulted in the clock-wisely rotating iso-capacity contours in the CaO–SiO₂–MnO–Al₂O₃–MgO system at 1873 K. The dissolution mechanism of sulfur in the MnO–containing calcium silicate melts can be explained not only by the difference in the structural role of Ca²⁺ and Mn²⁺ ions but also by the changes in the content of O^2– ions according to the silica content.