The present study investigated the relationship between sulfide capacity and the average nonbridging oxygens to tetrahedral cations (NBO/T) of CaO-SiO2-Al2O3-MgO slags. The sulfide capacity and NBO/T increase with increasing Al2O3/SiO2 ratio and X-CaO due to increasing basicity. The inflected dependence of sulfide capacity and NBO/T observed at a eutectic composition. Thermodynamic investigations revealed that the inflected dependence is caused by the effects of mixing enthalpy and entropy components in eutectic composition. Quantitative analysis of sulfide capacity and NBO/T revealed a proportional linear relationship between the two variables. Based on these experimental results, the corresponding structure-property relationships concerning sulfide capacity, viscosity, and stability are discussed in detail.