The electrolyte based on the molten KF-AlF3-NaF system was investigated for low-temperature aluminium electrolysis in laboratory scale. The current efficiency for aluminium deposition was determined from the amount of aluminium deposited after 4 h at a cathodic current density of 0.85 A•cm-2. The cryolite ratio of molar concentrations of alkali fluorides and aluminium fluoride KR = (NKF+NNaF)/NAlF3, was always retained constant, equal to 1.3 or 1.5. The cryolite-based melts contained 5 and 10 mass% NaF. The temperature was 750 °C for KR=1.3 and 800 °C for KR=1.5. Current efficiencies decreased with increasing content of NaF at cryolite ratios 1.3 and 1.5. The effect of NaF on the cathode process was studied. The contents of sodium and potassium in aluminium based on the KF-NaF-AlF3 melts were determined as a function of the cathodic current density in the range from 0.5 to 1.2 A•cm-2.