In Japanese steelworks, hot metal is now produced by scrap melting process. With this process removal of sulphur is very much handicapped because of very high sulphur levels (0.04 to 0.09 pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such disadvantages, the authors explored on the phase diagrams of {CaO-SiO2-Al2O3-MgO} slags, and this research revealed that those slags at 35 wt%-Al2O3 would be good candidates as reagents for the removal of sulphur from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, activities of FexO were determined by using solid-state electrochemical cells incorporating MgO-stabilized zirconia and MO + MoO2 reference electrode.