In modern steelmaking, manganese (Mn) is known as a versatile alloying element for various steel grades. Typical amounts of Mn are up to 1.5 wt.-% in low-alloyed carbon steels, around 3 to 6 wt.-% in medium Mn steels and more than 10 wt.-% in high Mn steels for special purposes. Mn increases strength and enhances toughness, ductility and weldability of steels in certain alloying ranges. In addition, Mn serves the purpose of binding residual sulfur (S) as manganese sulfides (Mn,Fe)S. In contrary, S is in general considered to be harmful in steel. S segregates in interdendritic liquid during the solidification of steel and causes hot tearing. In combination with Mn, sulfur leads to the formation of (Mn,Fe)S during the solidification in the continuous casting process, which is favorable to avoid the hot tear sensitivity. However, in the solid state, the precipitation of (Mn,Fe)S drastically decreases the ductility at elevated temperatures and thereby may induce crack formation at the surface. To assess the quality-related issues, the precise knowledge on thermodynamics of the Fe-Mn-S system and the prediction of (Mn,Fe)S stability in steel is crucial. The aim of this master¿s thesis was the experimental investigation and thermodynamic assessment of the ternary system Fe-Mn-S with emphasis on solidification of steel. Alloys with 0.5 wt.-% and 2 wt.-% Mn and up to 0.3 wt.-% S were investigated by Differential Scanning Calorimetry (DSC) regarding high-temperature phase equilibria and (Mn,Fe)S precipitation during cooling. Based on these results and experimental data from literature, thermodynamic modeling according to the CALPHAD (CALculation of PHase Diagrams) approach was carried out in the sub-systems Fe-Mn, Fe-Mn-C and Fe-Mn-S to create a database for the quaternary system Fe-Mn-S-C. For the liquid phase the Modified Quasichemical Model (MQM) in the pair-approximation was applied to formulate the solution¿s Gibbs energy. The MQM enabled to consider strong short-range-ordering (SRO) in the melt, which is typical for S-containing metal systems. The Compound Energy Formalism (CEF) was used for the descriptions of the solid solutions, e.g. austenite, ferrite, and various carbides. Several compounds were treated as stoichiometric to simplify the thermodynamic database.