The object of the present work was to clarify the properties of different secondary metallurgy ladle slag compositions for the production of heat-treated steels by investigating the phase equilibria with thermodynamic calculations and corresponding saturation experiments with the focus on high Al2O3- and MgO- bearing slags. Their impact on the dissolution behavior of product-critical oxidic inclusions of the type Al2O3 and MA-spinel was studied by In-situ High-Temperature-Laser-Scanning- Confocal-Microscope investigations at corresponding process temperature. The investigated ladle slag compositions showed a critical behavior in terms of a homogeneous liquid slag build up. This manly concerns the participation of solid MA-spinel out of the liquid slag phase. For this participation, the corresponding equilibrium concentrations were investigated as a function of temperature and FeOn and MnO concentration of the slag. The result showed that these components have little influence for industrial used ladle slag compositions. To investigate the influence of such solid MA-spinel bearing slags on the formation of exogenous nonmetallic inclusions in steels, different tracer materials were tested in laboratory scale. In this case, strontium oxide showed good results. In-situ dissolution experiments showed a great dependence between slag viscosities combined with corresponding saturation concentration of the slag to the associated dissolution times. The dissolution mechanism of MA-spinel and Al2O3 showed a different behavior. In the case of MA-spinel the dissolution mechanism showed a typical S-shaped form. The dissolution mechanism of Al2O3 particles was strongly depending on the slag viscosity. In this case the mechanism changed with increasing slag viscosity from “boundary layer controlled” to “diffusion in a stagnant fluid controlled” or an appropriate mixture of it. All in all, the results showed an enormous reduction of dissolution time and a homogeneous liquid slag formation by raising the CaO content of the investigated slags.