Proton conducting membranes are made of sulfonated films comprising
poly(arylene sulfide), an olefinic polymer, and an elastomer. They are
used in PEM fuel cells operating at temperatures above 95.degree. C., or
at low relative humidity. According to methods of the invention,
sulfonated poly(phenylene sulfide) (SPPS) films are provided with a wide
range of physical properties, which depend in part on the ion exchange
capacity of the films. In particular, the degree or level of sulfonation
can be tailored by adjusting reaction conditions such as temperature and
time.