To formulate the sulfur partitioning between basic steelmaking stags and metal in terms of the ionic treatments, the thermodynamic relation between equilibrium quotients and cationic concentrations proposed by Flood et al have been applied to the measured values. The results obtained are as follows. 1. Sulfur equilibrium is expressed by following equations. Stag/metal ; log {(N_o^(2-)/N_s^(2-) ·([a_s]/[a_o])} = -1.47N`_(ca)^(2+) - 1.92N`_(Fe)^(2+) - 3.15N`_(Mg)^(2+) Gas/slag ; log {{(N_o^(2-)/N_s^(2-) ·(P_s₂/P_o²)^½} = 2.35N`_(ca)^(2+) + 2.79N`_(Fe)^(2+) + 4.03N`_(Mg)^(2+) N_(A-) and N`_(M^+) are ionic Fraction and electrically equivalent fraction in the stags respectively. 2. The distribution ratio of sulfur between slags and metal is expressed by following equation. log {N_s^(2-)/[a_s]} = (-936/T + 1.375) + log C`_s - log [a_o] or, = (5214/T - 1.228) + log C`_s - log a_Fe_tO log C`_s { = N_s^(2-)·(P_o₂/P_s₂)^½} = - (2.35 N`_(ca)^(2+) + 4.03 N`_(Mg)^(2+)) + log(1 - N_(sio)₄- ) 3. The values of log { N_s^(2-)/[a_s] } are in good agreement with the measured values in the composition range of (CaO+MgO)/SiO₂> 1.7 for Fe_tO-SiO₂- CaO - MgO(sat) slags.