Desulphurization is a ladle treatment process in stainless steel production in which S is removed from the steel to the desired value. The removal of S is important to improve the mechanical properties, such as ductility and low-temperature toughness, and to increase the weldability. Steel manufacturers have been pushed to improve the quality of the steel due to consumers' requests for better mechanical properties of the steel. The desulphurization process is mainly done by slag treatment. The objective of this thesis is to investigate a new slag mixture called Calumet for the desulphurization of austenitic stainless steel. The investigation is done at different oxygen partial pressures, and desulphurization times and by changing the slag chemistry. The thesis is in collaboration with Aperam Genk., a global player in the production of stainless steel products. The steel and the slag are provided by Aperam Genk. FactSage 8.0 is used to predict thermodynamically the interaction of the steel and slag. Based on the thermodynamic calculations, lab tests are performed with the steel and different slag compositions at different oxygen partial pressures and desulphurization times in a high-temperature vertical tube furnace. The S content in the steel is measured by LECO analysis in Aperam Genk and the S content in the slag is measured by ICP-OES in MTM. Industrial tests were performed in Aperam Genk. Both FactSage calculations and experimental results show the sulphurization of the steel with Calumet slag. The S content in the slag is found to be the reason for the sulphurization of the steel. The amount of S in the steel increases with an increase in oxygen partial pressure and an increase in interaction time. The addition of CaO and CaF2 reduces the S in the slag which results in a decrease in the sulphurization of the steel. The pure Calumet slag with the current S content is therefore not recommended for the desulphurization of the steel. The results obtained from the present study are useful for the further improvement of the slag system in order to make it useful for the desulphurization of the steel.