A study has been made of the distribution equilibria between liquid iron and CaO-MgO-Fe_tO-SiO_2 slag saturated with MgO at 1600℃. The sulphide capacity C_s, the distribution ratio L_s of sulphur, the oxygen content [% 0] of liquid iron and the activity coefficient γ<Fe_tO> of Fe_tO were determined as functions of slag composition and theoretical optical basicity Λ, The main equations obtained are as follows: logC_s = 2.28 (N_<CaO> + 1.9N_<Fe_tO> + 0.1N_<MgO> - N<SiO_2>) -2.91log C_s = 1.06log N<Fe_tO> + 11.29Λ - 8.71 log L_s = 2.25 (N<CaO> + 1.9N<Fe_tO> + 0.1N<MgO> - N_<sio_2>) - log [% 0] - 2.02 log [% 0] = 0.736 log N_<Fe_tO> + 1.82Λ - 1.54 log γ_<Fe_tO> = -0.28 log N_<Fe_tO> + 1.86Λ - 0.93The validity and applicability of these equations have been discussed in comparison with the data reported by the other investigators.