Hyperbranched polymer (poly-la) with sulfonic acid groups at the end of chains was successfully synthesized. Interpenetration reaction of poly-la with a hyperbranched polymer with acryloyl groups at the end of chains (poly-1b) as a cross-linker afforded a tough electrolyte membrane. The poly-la and the resulting electrolyte membrane showed the ionic conductivities of 7 x 10(-4) and 8 x 10(-5) S/cm, respectively, at 150 degrees C under dry condition. The ionic conductivities of the poly-la and the electrolyte membrane exhibited the VTF type temperature dependence. And also, both poly-la and the resulting electrolyte membrane were thermally stable up to 200 degrees C. (c) 2005 Elsevier B.V. All rights reserved.