The content of sodium in molten aluminium during electrolysis of the molten system Na3AlF6-NaCl-Al2O3 was investigated at the temperatures 750, 890 and 1000degreesC. Simultaneously, the cathodic overvoltage was measured. It was found that the experimental data on the cathodic overvoltage agree well with the values predicted on the basis of the concentrations of sodium in molten aluminium. Saturation of molten aluminium with sodium was not achieved by electrolysis of a cryolite-based electrolyte.To achieve saturation of sodium in liquid aluminium the eutectic mixture NaF-NaCl was used as electrolyte. The experiments were carried out at the temperatures 710, 750, 800, 900 and 960degreesC, i.e. below and above the boiling temperature of sodium at atmospheric pressure. In this case, saturation of the molten aluminium with sodium was reached. The obtained data are treated by a thermodynamic model. (C) 2003 Elsevier Ltd. All rights reserved.