In order to be able to select the optimum composition of an electrolyte in the ZnCl2−NaCl−KCl system, the conductivity, surface tension and density were measured for a series of melts in the composition range 28–50 mol % ZnCl2, 0–62 mol% KCl and 0–62 mol % NaCl. It was found that the results could be correlated with the phase diagram and that the difference between the temperature of measurement and the liquidus or glass transition point was an important parameter in defining the properties. The optimum composition was found to be 40 mol % ZnCl2, 30 mol % KCl and 30 m