A slag-metal equilibrium study was carried out to investigate the effect of rare earth oxides on the sulphur partition between CaO-SiO₂ slags and carbon-saturated iron at 1500°C. The sulphur partition was increased with increase in Ce₂O₃ concentration in the slag. The oxygen potential of the system was found to be controlled by the Fe-FeO equilibrium. Sulphide capacities of CaO-SiO₂-Ce₂O₃ slags measured in the present study agreed well with the values predicted by the optical basicity method. It was tentatively concluded that Ce₂O₃ decreases the activity coefficient of SiO₂ in the CaO-SiO₂-Ce₂O₃ slag.