Poly(2,5-benzimidazole) (ABPBI) was synthesized in a novel polymerization medium containing CH3-SO3H and P2O5. 3,4-Diaminobenzoic acid, a monomer for ABPBI, is soluble in the medium and the polymerization was therefore performed in a homogeneous state. it produced polymer fibers, thus simplifying the work-up process. The membrane was cast directly from the polymerization mixture. Proton conductivities of the ABPBI membranes ranged from 0.02 to 0.06 S.cm(-1) above 100 degreesC, without humidification, and the tensile strength of the membrane was approximately 100 MPa.