Polymer electrolyte composite membranes were cast from the mixture of Nafion (R) ionomer and 2-substituted imidazole. The existing flexible hydrocarbon chain on 2-position of imidazole facilitates proton transfer in the membrane at elevated temperature. The formed composite membrane showed an increased glass transition temperature and improved mechanical property compared with plain Nafion (R) membrane. At temperature above 100 degrees C, the ionic conductivity of the composite membrane increases with the increase in temperature, reaching 5 x 10(-3) S cm(-1) at 160 degrees C under anhydrous condition. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 382-387, 2012