A method for preparing poly(2,5-benzimidazole) whereby 3,4-diaminobenzoic acid is polymerized using a dehydrating reagent containing P.sub.2O.sub.5 and CX.sub.3SO.sub.3H where X is H or F. The poly(2,5-benzimidazole) has good proton conductivity and low methanol permeability, and therefore can be used as a polymer electrolyte membrane for a fuel cell.