Phase equilibria in Fe-Mn-S and Fe-Cr-S ternary systems at 1 843 K were investigated experimentally, respectively. The main characteristic of these two systems at 1 843 K was confirmed to be a wide miscibility gap between two liquid phases: molten metal alloy phase and molten sulfide phase. Through metal/ sulfide equilibrium method, activity of constituents in sulfide phase were determined in Fe-Mn-S and Fe-Cr-S systems separately. The activity curves of constituents in sulfide phase were estimated by utilizing regular solution model.