A polymer electrolyte membrane obtained by subjecting a sulfonated polyarylene
membrane having an initial water content of 80-300 weight % to a hot-water treatment.
A composite polymer electrolyte membrane comprising a matrix made of a first sulfonated
aromatic polymer having a high ion exchange capacity, and a reinforcing material
constituted by a second sulfonated aromatic polymer having a low ion exchange capacity
in the form of fibers or a porous membrane.
