A tube-type cell was used to determine the electrical conductivity of low-melting electrolytes (AlF3-rich) by applying AC-techniques with a sine wave signal with small amplitude in the high-frequency range. The tested melt contained 55 mol % KF and 45 mol % AlF3 (molar ratio KF/AlF3 = 1.22) with and without the addition of 2 mass % alumina in the temperature range from 660 degreesC to 750 degreesC. The electrical conductivity K in this system can be described by the equation kappa/(Omega(1-.)cm(-1)) = 6.458 exp(-1858.91/T/K) - 0.01876(w(Al2O3)), where w(Al2O3) is the alumina concentration in the melt (mass %). Phase diagrams of the systems KF + AlF3 and KF + AlF3 + Al2O3 UP to 10 mol % Al2O3 for molar ratios KF/AlF3 = 2.35, 2.2, 1.85, and 1.22 (30, 31, 35, 45 mol % AlF3) were measured. In the melt with composition 55 mol % KF and 45 mol % AlF3, the solubility of alumina was aproximately 4 mol % (2.8 mass %) at 540 degreesC.