Abstract: The process of the present invention is a process for preparing 4,4'-dihydroxydiphenyl sulfone by subjecting phenol, and a sulfonating agent or phenolsulfonic acid to a dehydration reaction in the presence of a solvent, subsequently maintaining the reaction system at a temperature of between 100.degree. to 200.degree. C. and removing the solvent from the reaction system to cause the dehydration reaction to further proceed while allowing 4,4'-dihydroxydiphenyl sulfone to separate out and to subject dissolved 2,4'-dihydroxydiphenyl sulfone formed as a by-product to an isomerization reaction for converting the by-product to 4,4'-dihydroxydiphenyl sulfone, and eventually removing the solvent and the unreacted phenol from the system substantially entirely to complete the isomerization reaction, the process being characterized in that an aromatic sulfonic acid represented by the formula ##STR1## wherein X is a halogen atom, n is 0, 1 or 2, and m is 1 of 2 is caused to be present in the reaction system.

Abstract: A process for preparing diarylsulfone, the process comprising reacting chlorosulfonic acid with a mixture of (i) aromatic hydrocarbon represented by the formula ##STR1## wherein one of X and Y is a halogen atom and the other is a halogen atom or a hydrogen atom and (ii) aromatic sulfonic acid represented by the formula ##STR2## wherein X and Y are as defined above.

Abstract: In a process for producing 4,4'-dihydroxydiphenylsulfone by reacting phenol with sulfuric acid in the presence of a solvent, an improvement which comprises using at least a member selected from the group consisting of chlorobenzene, dichlorobenzene, trichlorobenzene, chlorotoluene, diethylbenzene, xylene, decalin, tetralin and tetrachloroethane as the solvent and removing said solvent progressively and finally substantially completely from the mixture while maintaining the reaction mixture at a temperature of about 160.degree. to about 200.degree. C. to effect precipitation of 4,4'-dihydroxydiphenylsulfone and isomerization of 2,4'-dihydroxydiphenylsulfone to 4,4'-dihydroxydiphenylsulfone.