1,3-1H-Dibenzimidazole-benzene (DBImBenzene) has been synthesized using phosphorus pentoxide-methanesulfonic acid (PPMA) as a solvent and dehydration agent and investigated as an additive (up to 2.0 wt.%) in sulfonated polysulfone (SPSf) membranes to promote proton conduction via acid-base interactions. The SPSf/DBImBenzene blend membranes with various DBImBenzene contents (0-2.0 wt.%) have been prepared and characterized by proton conductivity measurement and electrochemical polarization and methanol crossover measurements in direct methanol fuel cells (DMFCs). The blend membranes with DBImBenzene content of 0.5 and 1.0 wt.% show higher proton conductivities (3.4 and 2.9 x 10(-4) S/cm, respectively) than plain SPSf (2.4 x 10(-4) S/cm) even though the blend membranes have lower ion exchange capacity (0.81 and 0.75 mequiv./g, respectively) than plain SPSf (0.86mequiv./g). The blend membranes exhibit better electrochemical performance in DMFC than plain SPSf membrane due to an enhancement in proton conductivity through acid-base interactions and lower methanol crossover. (C) 2007 Elsevier B.V. All rights reserved.