Sulfonated poly(ether ether ketone)/polyethyleneimine (sPEEK/PEI) composite membranes were prepared to reduce the water uptake and methanol permeability of highly sulfonated PEEK membranes (>65%). Incorporation of small amounts of PEI reduced ionic cluster size via electrostatic complex formation between anionic sulfonic groups of the sPEEK and the cationic amine groups of the PEI, and thus affected membrane properties considerably. Ion cluster size decreased with increasing PEI concentration by small angle X-ray scattering pattern. Addition of 1 wt.% of PEI resulted in reduction of water uptake and methanol permeability by 30% at 60 degrees C and 85% at room temperature, respectively. The thermal and mechanical stabilities were also enhanced by formation of physical cross-linking induced by electrostatic interactions between acid/base polymers. Although proton conductivity was also reduced by PEI incorporation as a part of the sulfonic acid groups involved in ionic complex formation, its effect on proton conductivity was not as strong as on methanol permeability. (C) 2009 Elsevier B.V. All rights reserved.