Abstract: Process for the production of sodium chlorate by the electrolysis of sodium chloride contaminated with sodium sulphate wherein the sulphate concentration in the system is kept below a pre-selected level. A portion of the mother liquor remaining after crystallizing out some of the sodium chlorate product is selectively further cooled to effect crystallization of some of the sodium sulphate present in the liquor. An acceptable "steady-state" concentration of sulphate ion is established which prevents the deleterious excess sulphate ion build-up in the electrolytic system.