In steelmaking, one of the most important parameters for desulphurization is the oxygen potential of the system which can be expressed either by oxygen activity of the steel or by FetO activity of the slag. Removal of sulphur from the steel is enhanced when FetO activity of the slag and thus oxygen activity of the steel decreases. In this paper, ladle furnace slags of a low S steel quality currently produced in Eregli Iron and Steel Works Company (ERDEMIR), Turkey, were studied. Activity coefficients of FetO, gamma(FetO) were calculated with the relation obtained from the literature. The relation between %Decrease a(FetO) and %DeS (measured) was studied. The percentage of sulphur removal was also investigated using the data of the oxygen activity of liquid steel.