Chromium is an essential element in steelmaking industry. Steel slag inevitably contains some chromium and there is a risk of Cr emission from slag to the environment, however, in which form the emission of the element occurs and whether it is a hexavalent Cr is a concern. To investigate the conditions of hexavalent chromium formation in steel slags and its migration to the atmosphere, an estimation of Cr oxide's stabilities is carried out in this paper. Factsage 7.1 is applied to analyze the thermodynamic properties of Cr oxides with respect to the Cr6+ formation. An Ellingham diagram and standard Gibbs free energy change are proposed. Stability diagrams for BOF, EAF, and LF slag systems with various compositions are derived. Slag basicity is indicated as an influencing factor for Cr containing phase formation. Based on thermodynamic calculations Cr3+ phases are determined to be dominant for the given slags and no hexavalent Cr phases are found in the real multicomponent slag systems.