The general expressions for Raoultian activity coefficients in quaternary regular solutions have been derived. These equations allow calculation of activities as function of slag composition and temperature, provided six interaction energies are known for six constituent binary systems. The interaction energies for three binary solutions of the CaO-FeO, CaO-Fe0_1.5 and FeO-Fe0_1.5 systems have first been assessed, and the activity values of CaO (s), Fe_0.960 (1) and Fe01.33 (s) have been calculated for the CaO-FeO-Fe0_1.5 slags. They are in good agreement with the experimental data. The interaction energies in the binary oxidic solutions of the MeO-CaO, MeO-FeO and Me0-Fe0_1.5 (where MeO =Cu0_0.5, CoO, NiO, PbO, ZnO) have then been estimated from the heats of formation of binary compounds, liquidus curves, or terminal solid solubilities. The activity coefficients of minor oxides are then calculated at infinite dilution in ferrite slags, and compared with experimental data at 1523 K.