The modern metallurgy of iron and steel is first of all oriented in quality improvement, effectiveness and competitiveness of its production. Generally speaking, sulphur is considered a harmful element for steel quality, reason why all the technological steps are being taken in order to eliminate it from the metal bath. Molten systems of inorganic oxides forming a slag phase, form already during their genesis a complex heterogeneous system with many multiple reactions in the forming phase alone, and then subsequently individual components separately and in different combinations enter the interactions with other phases. Metallurgical slag qualities must be defined by the whole complex of physico-chemical characteristics, such as an oxidative ability, optical basicity, sulphide capacity up to slag fl uidity, its surface tension etc. The understanding of regulation of basic physico-chemical qualities of molten metals and slag depending on a chemical structure and a temperature has its importance at the level of the metallurgical process control. Presented paper deals with the possibilities how to exploit the sulphidic capacity for the desulphurisation evaluation in course of the metal reafi ning in the oxygen converter, and the during ladle treatment based on the set of the operational data. The thermodynamic model describes the slag/metal equilibria between O, and S in the molten steel and CaO, Al2O3, SiO2, FeO, MnO and S in the slag. If the oxygen activity, the activity coefficient of sulphur and the sulphide capacity at a given instant and a given position are known, the sulphur partition ratio can be evaluated at that position. The integral part of the work is the process of the pig iron desulphurisation. Magnesium has a high chemical affinity for sulphur and unlike other desulphurising reagents, dissolves in the iron-carbon melt with the result that its reaction with sulphur is homogeneous.