Abstract: A relatively concentrated and a relatively dilute aqueous salt solution are respectively passed through the anode and cathode chambers of a flow-through electrochemical cell having a porous membrane separating the two chambers. The fluid pressure in the cathode chamber is maintained at less than atmospheric pressure and at less than the fluid pressure in the anode chamber while a potential difference is applied across the anode and cathode. In this way, dangerous build-up of gaseous electrolysis products in the chambers, especially the anode chamber, is avoided.