A process for preparing 4,4'-dihydroxydiphenylsulfone, the process including heating 4,4'-dihydroxydiphenylsulfone and 2,4'-dihydroxydiphenylsulfone in the presence of an acid catalyst to isomerize the 2,4'-dihydroxydiphenylsulfone into 4,4'-dihydroxydiphenylsulfone, the process being characterized in that a suspension of 4,4'-dihydroxydiphenylsulfone and 2,4'-dihydroxydiphenylsulfone in a liquid dispersing medium is heated for isomerization while the liquid portion is distilled off or a crystal mixture of 4,4'-dihydroxydiphenylsulfone and 2,4'-dihydroxydiphenylsulfone is heated for isomerization to produce 4,4'-dihydroxydiphenylsulfone in the form of crystal powder.