An experimental and modelling study on hot metal desulfurization is considered. The study identifies the thermodynamic and kinetic factors that define the rate of permanent contact reaction in steady flow conditions. The study proceeds from defining the expressions for overall mass transfer rate of desulfurization and uses experimental data to identify the mass transfer coefficients. The study draws an estimate of the model parameter distributions using a real–coded genetic algorithm and a Metropolis Hastings’ algorithm. The results of this study and the literature data indicate that the Na2O content (5–20 wt–%) of the slag has a great potential for improving the desulfurization efficiency via the increase of the rate of the permanent contact reaction. The desulfurization efficiency increased 5 unit–% per unit–% increase in slags Na2O content (below 10 wt–%). This can be implied to potentially decrease the desulfurization reagent consumption in powder injection.