The approximate validity of the regular solution model for canons was examined to formulate the thermodynamic properties of steelmaking slags and to express quantitatively the equilibrium reaction between slag and metal. 1) The regular solution model was satisfied for multi-component stags such as Fe_tO-SiO₂-MgO, Fe_tO-SiO₂-CaO-MgO and Fe_tO-SiO₂-CaO-MgO-P₂O_5 systems, and the following values were obtained as interaction energies between cations. α_(15)(Fe^(2+)∼Mg^(2+))=3.070㎈, α_(25)(Fe^(3+)∼Mg^(2+))=-5,620㎈, α_(35)Si^(4+)∼Mg^(2+))=-30,500㎈, α_(45)(Ca^(2+)∼Mg^(2+))=4,500㎈, α_(36)(Si^(4+)∼P^(5+))=11,000㎈, α_(46)(Ca^(2+)∼P^(5+))=-125.000㎈, α_(56)(Mg^(2+)∼P^(5+))=-85,000㎈ 2) It was confirmed that applying the values of interaction energy to the regular solution model had enough certainty to estimate the oxygen content in liquid iron within the accuracy of ±10% over the wide range of slag compositions from basic to acidic stags. 3) Simple regular solution approximation was less favorable to apply for the phosphorus distribution between liquid iron and stags. It was proposed that a further modification of the model considering composition dependence of α_(ij) should be necessary to express the phosphorus equilibrium with a mathematical formula.