We synthesized thermally stable proton-conducting polymers composed of poly(benzimidazole) (PBI) and strong acids. PBI films were doped with strong acids such as phosphoric, sulfuric, and hydrochloric acid by immersing them into a mixed solution of strong acid and methanol. We found that the PBI films and strong acids formed polymer complexes because they showed the acid-base or the hydrogen bonding interactions between imidazole groups of PBI and acid molecules. The PBI/H3PO4 complexes were thermally stable up to 500 degrees C, and the proton conductivity of anhydrous complexes reached 10(-5) S cm(-1) at 160 degrees C. (C) 2000 Elsevier Science Ltd. All rights reserved.