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.