The effect of oxide additives, such as BaO, Li₂O, K₂O and Na₂O in CaO-CaF₂-SiO₂ slags on the equilibrium phosphorus distribution ratios, L_p between molten slags and Fe-C_(sat·)-P melts were studied at 1300℃. The effect of BaO addition on the increase in the L_p is more significant when it is added to the lower basicity slag. The small addition of Li₂O K₂O and Na₂O in CaO-CaF₂-SiO₂ slags significantly increases the L_p. The stability of Li₂O in slag is much higher than K₂O and Na₂O during the dephosphorization reaction. The correlation between the phosphate capacities of slags containing alkali metal oxides does not show the linear relationship with their optical basicities. The alkali metal oxides decrease the activity coefficient of phosphate ions in slag drastically, and thereby increase the dephosphorization capability of CaO based slags.