The thermodynamic behavior of MnO in the CaO–SiO₂–MnO slag system of lower basicity to understand the smelting reduction of silicomanganese was investigated. Experimental results indicated that the activity coefficient of MnO was not affected by MnO content less than 30 mol%. However, it did in fact increase as the MnO content increased beyond 30 mol% because the activity coefficient of MnO is closely associated with the silicate structure. The activity coefficient of MnO also increased with increasing basicity of slag and MgO content because of the free O²⁻ ions that are provided into the slag. On the other hand, the effect of the Al₂O₃ content on the activity coefficient of MnO was analyzed that the structure in the CaO–SiO₂–MnO–Al₂O₃ system changes from that of a Si–O–Si linkage to that of Si–O–Al and Al–O–Al linkages as the content of Al₂O₃ increases. Structural considerations concerning the effect of slag composition on the activity coefficient of MnO are discussed in detail using Fourier transform infrared (FT-IR) spectroscopy. Finally, it is shown that manganese recovery can be increased by increasing the activity coefficient of MnO in the slag.