An electrolysis process for chlorine-caustic production using a mobile film of mercury as the cathode, and a cation permeable membrane to separate the electrodes, the film of mercury continuously flowing across one side of the membrane. The membrane is impervious to the passage of the mercury therethrough for preventing short circuits between the mercury cathode and the anode, the anode being preferably a grid. The membrane is disposed against the anode to avoid breakage of the membrane due to the pressure of the film of mercury. Preferably, the membrane is positioned in a vertical arrangement and the film of mercury flows in an upward direction across the membrane. Anolyte and cathode sections of the electrolytic cell may be stacked in a matching alternating side-by-side arrangement with each of the sections being bifrontal.