In a comprehensive study of the literature, the conductivity values of metallurgical slags obtained using different methods and under various test conditions were compared. The differences in the various conductivity values are discussed here with respect to their cause, allowing conclusions to be drawn on the advantages and disadvantages of the testing methods used. We chose the optimum equipment and method based on these conclusions. Accordingly, a Tammann furnace with a maximum temperature of 1650 °C was modified to measure conductivity. Results from the literature are qualitatively and quantitatively compared with the practical results of multiple investigations of the slags. The influence of MgO on the conductivity of multi-component slags has not been extensively investigated to date, meaning that there is as yet no consensus on the effects of MgO. We therefore investigated this influence in a special series of investigations where the CaO in ESR slags was systematically replaced by MgO.