A series of sulfonated poly(phthalazinone ether sulfone) (SPPES) membranes with various sulfonation degrees have been fabricated to reduce the methanol crossover in direct methanol fuel cells (DMFCs). The SPPES membranes were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) measurements, electrochemical methods, thermal analysis and sorption studies. The SPPES membranes exhibit high proton conductivity. The methanol permeabilities of these membranes are at least one order of magnitude lower than Nalion (R) 115 membrane. The SPPES membrane appears to be an excellent candidate for the DMFCs application. (c) 2007 Elsevier B.V. All rights reserved.