Process for the electrolysis of alkali metal chloride solutions

A process for electrolyzing an aqueous alkali metal chloride solution in a membrane cell which contains an anode chamber with the anode and a cathode chamber with the cathode, the two chambers being separated from one another by a cation exchanger membrane, in which the cathode is porous and foil-like, the cathode and cation exchanger membrane form the actual cathode chamber filled with catholyte, the cathode and cell wall form a gas space, water is fed into the cathode chamber and alkali metal hydroxide solution is withdrawn from the cathode chamber, hydrogen is withdrawn from the cathode chamber and gas space, aqueous alkali metal chloride solution is fed into the anode chamber and gaseous chlorine, together with depleted alkali metal chloride solution, is withdrawn from the anode chamber, and a direct voltage which is at least the same as the decomposition voltage is applied to the cathode and anode, which comprises establishing a higher pressure in the cathode chamber than in the gas space during the electrolytic process, and wherein the pressure in the catholyte chamber is 10 mbar to 0.5 bar higher than in the gas space, and no oxygen is supplied to the cathods.