An aromatic polymer film substrate, or a grafted aromatic polymer film
substrate having a monomer introduced therein as graft chains is
irradiated with ionizing radiation to impart a crosslinked structure. The
aromatic polymer film substrate or the grafted aromatic polymer film
substrate, provided with the crosslinked structure, is directly
sulfonated to obtain a crosslinked aromatic polymer electrolyte membrane.
The crosslinked aromatic polymer electrolyte membrane has low water
uptake, high proton conductivity, low methanol permeability, high
chemical stability, and excellent mechanical characteristics. A method
for producing the crosslinked aromatic polymer electrolyte membrane is
also provided.