Abstract: The invention concerns the combined production of sodium peroxide disulphate and soda lye from sodium sulphate. According to the invention, electrolysis is carried out in at least one two-chamber electrolysis cell with cathode and anode chambers separated by a cation-exchanger membrane. The electrolysis temperature is set at between 30 and 70.degree. C., a sodium sulphate solution which is at least 75% saturated at this temperature is introduced into the anode chamber, the sodium ion concentrations are maintained in the range of between 4.5 and 6.0 mol/l and soda lye is extracted from the cathode chamber.