The direct sulfonation of a poly-[(1-(4,4-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI-00) in solution is described. The sulfonation is carried out directly in sulfuric acid under mild reaction conditions. In PBI-00, the presence of electron donor ether bridges confers particular properties that allow mild and inexpensive routes for direct backbone sulfonation by electrophilic substitution. PBI-00 is a particularly interesting and versatile polymer from the point of view of modification, since it offers four rings per repeat unit at which sulfonation may occur. The degree of sulfonation of PBI-OO depends on the duration and, particularly, the temperature of the reaction medium. H-1 NMR spectroscopy has been used to determine the site and extent of sulfonation. Membranes of sulfonated PBI-00 display ionic conductivity up to 0.08 S cm(-1) at 120 degrees C and 100% relative humidity depending on the degree of sulfonation. Mechanical properties are described. (c) 2008 Elsevier B.V. All rights reserved.