Methanol permeability of directly copolymerized 4,4'-biphenot based disulfonated poly(arylene ether sulfone) copolymers (BPSH) was investigated with reference to utility as a proton exchange membrane (PEM) for direct methanol fuel cells (DMFC). Water uptake and dynamic mechanical analysis were coupled with previous observations that the PEM can have two functional morphological regimes, which depend on the degree of disulfonation (copolymer composition), acidification method, and hydrothermal treatment. The two regimes are observed by AFM to represent: (1) a "closed" structure where the hydrophilic copolymer chain segments essentially aggregate as isolated domains; or (2) an "open" structure where the domain connectivity of the hydrophilic phase of the copolymers is achieved. It was demonstrated that methanol permeability (25 degreesC) of the copolymers abruptly increased at copolymer compositions and processing conditions that influenced the membrane morphology to change from a closed to a much more open structure. The activation energy in the closed structure regime, similar to20 kJ/mol, was about 35% higher than that in the open regime, similar to15 kJ/mol. The BPSH copolymers had higher selectivity (i.e. proton conductivity/permeability) than Nation because of their remarkably lower methanol permeability, suggesting these materials hold promise for improved DMFC performance. Selectivity increased with the degree of disulfonation in closed structures, but decreased in the open structure regime. It is suggested that the optimum concentration of proton conducting groups for DMFC should be observed at or near the percolation threshold. (C) 2004 Elsevier B.V. All rights reserved.