Abstract: HSC 6.1 Chemistry software (Outokumpu) and a simplex–lattice experiment design are employed in thermodynamic modeling of the equilibrium boron distribution between steel containing 0.1% C, 0.54% Si, and 0.01% Al (wt % are used throughout) and CaO–SiO2–B2O3–25%Al2O3–8%MgO slag over a broad range of chemical composition at 1500 and 1600°C. For each temperature, mathematical models (in the form of a reduced third-order polynomial) are obtained for the equilibrium boron distribution between the slag and the molten metal as a function of the slag composition. The results of simulation are presented as graphs of the composition and equilibrium distribution of boron. The slag basicity has considerable influence on the distribution coefficient of boron. Experimental data practically confirm the results of thermodynamic modeling.