The present invention provides a method for sanitising water in a swimming pool or the like, which method uses sources of ionic chlorine at significantly lower levels than conventional systems. The method comprises the steps of (i) forming, in the swimming pool water, an electrolyte solution containing from 500 ppm to 9000 ppm of a soluble magnesium halide salt, (ii) treating the electrolyte solution in an electrolytic halogenation cell to form an aqueous solution of hypohalous acid, and (iii) returning the water so treated back to the swimming pool. A mixture of magnesium, potassium and sodium chloride salts with small quantities of a soluble alkaline earth metal bromide, zinc halide, ascorbate, and/or zinc ascorbate may be particularly effective in the sanitisation process.
