The Fe-C-Cr-Mn-Ni-O system is of fundamental importance when describing the influence of oxygen on high alloyed steels. Both solid and liquid phases are of great interest: The solid phases regarding oxidation processes like the formation of oxide layers, inner oxidation, sintering processes and high temperature corrosion. The liquid phase is of interest concerning the interaction between steel and its slag in a metallurgical context. In this thesis the thermodynamic properties of this system is described using the Calphad technique. The main idea of the Calphad technique is to describe the Gibbs energy of all phases in the system as a function of temperature, pressure and composition using appropriate thermodynamic models. When thermodynamic descriptions of all phases taking part in the system are modelled and described in a database, the equilibrium state could be calculated with a software that minimizes the total Gibbs energy. Models within the compound energy formalism are used for all solution phases, among them the ionic two-sublattice liquid model, to describe both the metallic and oxide melts. All simple spinels (Cr3O4, FeCr2O4, Fe3O4, FeMn2O4, Mn3O4, MnCr2O4, NiCr2O4, NiFe2O4, NiMn2O4) within this system are described using a four-sublattice model. In this thesis several binary and ternary systems have been assessed or partly reassessed. The Fe-C-Cr-Mn-Ni-O database achieved can be used with an appropriate thermodynamic software to calculate thermodynamic properties, equilibrium states and phase diagrams. In general, the agreement between calculated and experimental values is good. / QC 20100723