Abstract: A prediction model of sulphide capacity in CaO-SiO₂-MgO-Al₂O₃ slag systems was developed based on the ion and molecule coexistence theory (IMCT) of slags, i. e., IMCT-CS₂-model. The predicted sulphide capacity by the IMCT-CS₂-model was compared with the measured one from experiments and the predicted one by other five prediction models of sulphide capacity. The predicted sulphide capacity of CaO-SiO₂-MgO-Al₂O₃ slags at 1 773 K by the developed IMCT-CS₂-model has a higher accuracy than the measured one as well as the predicted one by other sulphide capacity models. The developed IMCT-CS₂-model can calculate not only the total sulphide capacity of the slags but also the respective sulphide capacity of free CaO and MgO in the slags. Increasing the Al₂0₃ content from 10% to 17%, increasing the CaO content from 38% to 44% and decreasing the MgO content from 12% to 4% can improve the contribution ratio of free CaO from 97% to 99% and decrease the contribution ratio of free MgO from 3% to about 1% to the total sulphide capacity of CaO-SiO₂-MgO-Al₂O₃ slags.