The desulfurization of hot metal using Na2O-containing fluxes in the hot metal pretreatment stage is of increasing interest. To understand the sulfide capacity of the fluxes, preliminary thermodynamic modeling of the Na2O-FetO and Na2O-FetO-SiO2 systems was performed. The sulfur dissolution in the Na2OFetO-SiO2 melts was then modeled using the two-sublattice Modified Quasichemical Model, which can take into account the short-range-ordering and the reciprocal exchange reaction of cations and anions in the oxy-sulfide slags. Experimental sulfide capacity data were satisfactorily reproduced in the modeling. Using the developed thermodynamic database and the available liquid Fe database in FactSage, thermodynamic calculations for the desulfurization process of hot metal were performed.