Abstract: A process is provided for the crystallization of bis-(4-chlorophenyl) sulphone from a reaction mixture in which it is formed by reacting chlorobenzene sulphonic acid and chlorobenzene which comprises (i) forming a crystallization liquor from the reaction mixture by adjusting the weight ratio of chlorobenzene to chlorobenzene sulphonic acid of the reaction mixture so as to maximize the quantity of bis-(4-chlorophenyl) sulphone which can crystallize from the liquor at a desired crystallization temperature, and (ii) allowing the bis-(4-chlorophenyl) sulphone to crystallize from the crystallization liquor at the temperature.