New sulfonated poly(imidoaryl ether sulfone) copolymers derived from sulfonated 4,4'-dichlorodiphenyl sulfone, 4,4'-dichlorodiphenyl sulfone, and imidoaryl biphenol were evaluated as polymer electrolyte membranes for direct methanol fuel cells. The sulfonated membranes were characterized with Fourier transform infrared spectroscopy, thermogravimetric analysis, and proton nuclear magnetic resonance spectra. The state of water in the membranes was measured with differential scanning calorimetry, and the existence of free water and bound water was discussed in terms of the sulfonation level. The 10 wt % weight loss temperatures of these copolymers were above 470 degrees C, indicating excellent thermooxidative stability to meet the severe criteria of harsh fuel-cell conditions. The proton conductivities of the membranes ranged from 3.8 x 10(-2) to 5 x 10(-2) S/cm at 90 degrees C, depending on the degree of sulfonation. The sulfonated membranes maintained the original proton conductivity even after a boiling water test, and this indicated the excellent hydrolytic stability of the membranes. The methanol permeabilities ranged from 1.65 x 10(-8) to 5.14 x 10(-8) cm(2)/s and were lower than those of other conventional sulfonated ionomer membranes, particularly commercial perfluorinated sulfonated ionomer (Nafion). The properties of proton and methanol transport were discussed with respect to the state of water in the membranes. (c) 2005 Wiley Periodicals, Inc.