A slag-metal equilibrium study was carried out to investigate the effect of oxygen activity on the sulfur partition for CaO-Al2O3-SiO2-MgO slag. The sulfide capacity C-s prediction models by Sosinsky-Sommerville based on the optical basicity Lambda and the KTH model in terms of the defined interaction coefficient of the component i to j xi(i-j)(Interaction) is used in this work for both a comparison and an estimation of the sulfur partition ratio between ladle-treated slag and liquid steel. From the obtained results, it was shown that the Sosinsky-Sommerville optical basicity approach gives higher values for the sulfur partition ratio (L-s) compared with the KTH model.