Polysulfones and polyphenylsulfones having pendant phenyl groups with sulfonic acid units have been prepared by lithiation of the respective polymer, followed by reaction with 2-sulfobenzoic acid cyclic anhydride. The resulting ionomers were cast into membranes and properties such as thermal stability, ion-exchange capacity, water sorption and proton conductivity were evaluated. These membranes proved to have a high thermal stability, with a decomposition temperature between 300 and 350degreesC, and a high proton conductivity, 60 mS/cm at 70degreesC for a polyphenylsulfone with 0.9 sulfonic acid group per repeating unit measured at 100% relative humidity. Moreover, some of the membranes endured immersion in water at temperatures ranging from 20 to 150degreesC without swelling extensively, and therefore kept their mechanical stability under these conditions. It was also shown that these membranes retained a high conductivity up to 150degreesC under humidifying conditions. The combination of properties make these membranes potential candidates for fuel cells operating at temperatures above 100degreesC. (C) 2004 Elsevier Ltd. All rights reserved.