Sulphur distribution ratios have been determined between carbon-saturated iron and Na2O-SiO2, Na2O-SiO2-CaO and Na2O-SiO2-CaO-CaF2 slags at 1350°C. The experiments were performed in graphite crucibles under a carbon monoxide atmosphere. Sulphide capacities were calculated based on the sulphur distribution ratios. Additions of Na2O increase the sulphide capacity of silicate as well as lime-based slags significantly and the Na2O-equivalent of CaO was determined as O.30. CaO-Na2O-SiO2-CaF2 slags possess high sulphide capacities as well as low melting points, and can consequently be utilized for hot-metal treatment at relatively low temperatures. However, additions of CaF2 decrease the sulphide capacity of these slags and consequently only enough CaF2 should be used to ensure that the slags remain liquid during the desulphurization treatment. © 1993, The Iron and Steel Institute of Japan. All rights reserved.