The interaction coefficient of phosphorus in carbon saturated Fe-Mn, Mn-Si alloys has been measured at temperatures of 1573 to 1673K using a chemical equilibration technique between BaO-BaF₂ fluxes and alloys. A slight decrease in the activity coefficient of phosphorus in Fe-Mn-C_(satd), alloys was observed with increasing manganese content, reflecting a stronger interaction between manganese and phosphorus than that between iron and phosphorus. We also found that interaction coefficient between silicon and phosphorus in the Mn-Si-C_(satd), alloys ε^(Si)_(P C_(satd)), was found to be 3.20, 4.16, 5.26 at 1573, 1623 and 1673K, respectively.