The ESR process is characterized by its slag, which has to meet the criteria defined by its chemical and physical properties. One of the most important physical properties is the specific electrical conductivity which is of basic interest for the further development of new ESR slags. The specific electrical conductivity of CaF2-based slags was measured by the four electrode-method using a specially constructed measurement cell. The main focus of this investigation was the CaF2-CaO-MgO-system. In addition the influence of basic oxides on selected slag compositions were investigated. The measured conductivity was compared with results calculated from an extended empirical equation originally determined by Ogino. Boundary conditions were defined for the newly integrated components. Results show excellent reproducibility for the integration of MgO slags into the mathematical function. For the other less investigated oxides only first approximations were possible. Experiments were also made on an industrial scale using a pressureised electroslag remelting furnace to validate the transferability of the laboratory results. This was achieved by remelting of a plastic mould steel with selected slag compositions. CaF2-CaO-MgO-slags in comparison with pure CaF2 slags result in a lower energy consumption but there were no additional advantages when compared to the standard slags of the CaF2-CaO-Al2O3-system.