An ultrathin polyetheretherketone (PEEK) film was modified by thermal grafting of divinylbenzene (DVB) followed by radiation grafting of ethyl styrenesulfonate (ETSS), to develop a polymer electrolyte membrane (PEM) for use in direct methanol fuel cells (DMFCs). The DVB-grafting yield by thermal polymerization was less than 7%, but was sufficient to enhance the subsequent ETSS-grafting. The ETSS units were converted to styrenesulfonic acid (SS) units by hydrolysis in water at 95 degrees C, resulting in an ultrathin PEEK-g-DVB-g-SS PEM. FT-IR and TG analysis as well as measurements for the ion exchange capacity (IEC), water uptake, proton conductivity, mechanical strength, and methanol permeability were performed. Furthermore, this new PEM was tested in a DMFC, and a high maximum power density of 110 mW cm(-2) at 95 degrees C was confirmed. Therefore, we conclude that this modified PEEK film can be used as the PEM for the application in DMFCs. (C) 2009 Elsevier B.V. All rights reserved.