A review is presented of the published studies of the use of molten salt electrolysis in the refining of metals. The expression of the results of transport measurements in fused salts is discussed and the friction coefficient formalism is described. A theoretical analysis of the behaviour of an anode impurity in electrolysis is given and a solution is obtained for certain simple boundary conditions. A refinability parameter β is obtained which describes the behaviour of the impurity. If β is less than i the impurity is concentrated in the anode; if β is greater than 1 the impurity is concentrated in the cathode. It is shown that there is a limit to the refining that can be achieved in any one system by raising the current density. At the limiting current density the refining ratio - the ratio of impurity in the cathode to impurity in the anode - is given by the parameter β. An approximate value of β may be readily obtained from thermodynamic and electrical conductivity data generally available in the literature. The analysis has been extended to cover the effects of additions of diluting salts, whose cations do not take part in the electrode processes and a solution has been obtained for a simple batch refining process. The transport of copper, silver and cadmium as anode impurities in lead has been studied, the electrolyte containing the mixed chlorides. The effect of adding sodium chloride to the melt has been investigated in the lead-copper and lead-cadmium systems. The predictions of the theory are confirmed in the lead-copper and lead-silver systems although there are some differences between the predicted and experimental refining ratios. These differences are due to approximations made in estimating β due to lack of data in the literature. The lead-cadmium system shows a considerable deviation from the predicted value and it is suggested that this is due to solution in the melt of cadmium deposited at the cathode. The investigation has indicated that the ionic species responsible for the transport of copper and cadmium are Cu²₂+ and CdC1+ respectively.