Novel proton conducting membranes, sulfonated polyethersulfone Cardo (SPES-C), were prepared with concentrated sulfonic acid at room temperature. The degree of sulfonation was controlled by reaction time. Their proton conductivity and methanol permeability as a function of temperature were investigated. The SPES-C membranes with 70% DS were still not water soluble and had low degree of swelling. With the level of 70% sulfonation, proton conductivity was 0.011 S/cm at 80degreesC. 0.0338 S/cm at 100degreesC, which approached that of Nafion(R) 115 membrane at the same conditions. Methanol permeability of SPES-C membranes was considerably smaller than that of Nafion(R) 115 membrane over the temperature 25-80degreesC. (C) 2004 Published by Elsevier B.V.