Sulfonated polysulfones (SPSf) with different degree of sulfonation (DS) have been synthesized and evaluated as proton exchange membranes in direct methanol fuel cell (DMFC). The membranes have been characterized by ion exchange capacity (IEC), proton conductivity, liquid uptake, and single DMFC polarization measurements. The proton conductivities of the SPSf membranes increase with increasing sulfonation, but are lower than that of Nafion 115. Within the range of sulfonation of 50-70%, the SPSf membranes exhibit better performances in DMFC than Nation 115 at lower methanol concentrations (1 M) despite lower proton conductivities due to suppressed methanol permeability and crossover. However, the performances of SPSf membranes at higher methanol concentrations (2 M) are inferior to that of Nafion 115 at current densities higher than about 50 mA cm(-2) as the suppression in methanol crossover could not quite compensate for the lower proton conductivities. (c) 2005 Elsevier B.V. All rights reserved.