A process for producing 4,4'-dichlorodiphenyl sulfone, comprising: (I) reacting thionyl chloride, chlorobenzene and aluminum chloride in a molar ratio of thionyl chloride : chlorobenzene : aluminum chloride of 1 : (6 to 9) : (1 to 1.5) at a temperature from 0 to below 20°C, forming an intermediate reaction product and hydrogen chloride; (II) mixing aqueous hydrochloric acid and the intermediate reaction product at a temperature from 70 to 110°C to obtain an organic phase comprising 4,4'-dichlorodiphenyl sulfoxide and an aqueous phase; (III) cooling the organic phase comprising the 4,4'-dichlorodiphenyl sulfoxide to a temperature below the saturation point of 4,4'-dichlorodiphenyl sulfoxide to obtain a suspension comprising crystallized 4,4'-dichlorodiphenyl sulfoxide; (IV) solid-liquid-separation of the suspension to obtain a residual moisture comprising solid 4,4'-dichlorodiphenyl sulfoxide comprising crystallized 4,4'-dichlorodiphenyl sulfoxide and solvent, and mother liquor; (V) washing the residual moisture comprising solid 4,4'-dichlorodiphenyl sulfoxide with a carboxylic acid to obtain carboxylic acid-wet 4,4'-dichlorodiphenyl sulfoxide; (VI) reacting the carboxylic acid-wet 4,4'-dichlorodiphenyl sulfoxide and an oxidizing agent in a carboxylic acid as solvent to obtain a reaction mixture comprising 4,4'-dichlorodiphenyl sulfone and carboxylic acid; (VII) separating the reaction mixture comprising 4,4'-dichlorodiphenyl sulfone and carboxylic acid into a residual moisture comprising 4,4'-dichlorodiphenyl sulfone as crude product and a liquid phase comprising carboxylic acid. (VIII) optionally working up the residual moisture comprising 4,4'-dichlorodiphenyl sulfone. (I) reacting thionyl chloride, chlorobenzene and aluminum chloride in a molar ratio of thionyl chloride : chlorobenzene : aluminum chloride of 1 : (6 to 9) : (1 to 1.5) at a temperature from 0 to below 20°C, forming an intermediate reaction product and hydrogen chloride; (II) mixing aqueous hydrochloric acid and the intermediate reaction product at a temperature from 70 to 110°C to obtain an organic phase comprising 4,4'-dichlorodiphenyl sulfoxide and an aqueous phase; (III) cooling the organic phase comprising the 4,4'-dichlorodiphenyl sulfoxide to a temperature below the saturation point of 4,4'-dichlorodiphenyl sulfoxide to obtain a suspension comprising crystallized 4,4'-dichlorodiphenyl sulfoxide; (IV) solid-liquid-separation of the suspension to obtain a residual moisture comprising solid 4,4'-dichlorodiphenyl sulfoxide comprising crystallized 4,4'-dichlorodiphenyl sulfoxide and solvent, and mother liquor; (V) washing the residual moisture comprising solid 4,4'-dichlorodiphenyl sulfoxide with a carboxylic acid to obtain carboxylic acid-wet 4,4'-dichlorodiphenyl sulfoxide; (VI) reacting the carboxylic acid-wet 4,4'-dichlorodiphenyl sulfoxide and an oxidizing agent in a carboxylic acid as solvent to obtain a reaction mixture comprising 4,4'-dichlorodiphenyl sulfone and carboxylic acid; (VII) separating the reaction mixture comprising 4,4'-dichlorodiphenyl sulfone and carboxylic acid into a residual moisture comprising 4,4'-dichlorodiphenyl sulfone as crude product and a liquid phase comprising carboxylic acid. (VIII) optionally working up the residual moisture comprising 4,4'-dichlorodiphenyl sulfone. <EOS>