A swimming pool water sterilizer and the method involved in the operation are disclosed employing a two electrode compartment electrolytic cell with said compartments being separated by a cation selective membrane. A concentrated alkali metal chloride salt solution is fed to the anode compartment wherein on application of a decomposition voltage there is formed therein essentially chlorine gas and hypochlorous acid. The migration of water from the anode compartment to the cathode compartment as a result of the solvation of the alkali metal ions (Na ) which passes to the cathode compartment through the cation membrane allows the formation of a caustic solution and hydrogen gas within the cathode compartment. The effluents resulting from both compartments are fed into the recirculated pool water for sterilization. The pH of said water can be controlled by discharging to waste any excess basic catholyte at predetermined intervals at pre-set periods of time.