Abstract: Apparatus is provided for the chlorination of water in a swimming pool where the water includes a soluble chlorine ion containing electrolyte. The apparatus comprises a housing which is mounted on the side wall of a swimming pool under the water level. The housing has a plurality of electrodes located therein, the electrodes being connected to a low voltage power supply. The housing has a plurality of openings from the outside to the interior, whereby water is able to pass in and out of the housing, where electrolysis of the soluble chlorine ions takes place when they come into contact with the electrodes. The water which has been effectively chlorinated passes out of the housing to be replaced by other water with the soluble chlorine ion still in solution.