Proton exchange membrane fuel cells (PEMFC) are promising new power sources for vehicles and portable devices. Membranes currently used in PEMFC are perfluorinated polymers such as Nation. Even though such membranes have demonstrated good performances and long-term stability, their high cost and methanol crossover makes them unpractical for large-scale production. Sulfonated aromatic poly(ether ether ketones) (S-PEEKS) based membranes have been studied due to their good mechanical properties, thermal stability and conductivity. In this study, PEEK membranes directly prepared from the sulfonated monomer were evaluated for possible fuel cell applications by determining the degree of sulfonation, water swelling, proton conductivity, methanol diffusivity and thermal stability. As synthesized S-PEEK membranes exhibit conductivities (25 degreesC) from 0.02 to 0.07 S/cm, water swelling from 13 to 54%, ion-exchange capacities (IEC) from 0.7 to 1.5 meq/g and methanol diffusion coefficients from 3 x 10(-7) to 5 x 10(-8) cm(2)/s at 25 degreesC. These diffusion coefficients are much lower than that of Nafion (2 x 10(-6) cm(2)/s), making S-PEEK membranes a good alternative to reduce problems associated with high methanol crossover in direct methanol fuel cells. (C) 2004 Elsevier B.V. All rights reserved.