A process for the preparation of 4,4-dichlorodiphenyl sulfone by heating a mixture of (a) chlorobenzene, (b) chlorosulfonic acid or sulfur trioxide, and (c) thionyl chloride or phosgene to temperatures up to 220.degree. C., more than the stoichiometric amount (based on the amount of chlorobenzene) of chlorosulfonic acid or sulfur trioxide and less than the stoichiometric amount of thionyl chloride or phosgene being employed.