An electrolyser for disinfectant production comprises a hollow cylindrical cathode 20 through which fresh water is channelled. A cathode chamber is defined between the outer surface of the cathode and an inner surface of a coaxial diaphragm 80 and an anode chamber 32 is defined between the outer surface of the diaphragm and a cylindrical anode 50 coaxially surrounding it. A cover mixer 30 at the upper end of the cell has an internal annular mixing channel 33 in communication with the upper end of the anode chamber, with a water inlet 31 and an outlet 34 from which a flow 6 of disinfecting liquid is discharged at a predetermined rate. In use, an aqueous sodium chloride solution is fed to the anode chamber at a flow rate 16 to 20% of the product flow rate 6, and water or aqueous sodium chloride is fed through the cathode chamber. Cooling water passed through the inside of the cathode is then fed through inlet 31 to the mixing channel to bring the product solution to the required flow rate and pH level. Hydrogen is channelled to an outlet 36 from within the cathode chamber.
