The synthesis and characterization of a series of novel acid monomers based on sulfonimides with different acidities are described. Copolymerization of the acid monomers with acrylonitrile was carried out, and the proton exchange membranes were cast from solutions of the resulting copolymers. Flexible, mechanically strong, and transparent membranes with different equivalent weights could be prepared. The membranes with lower equivalent weights and higher acidities exhibited higher proton conductivity under humidifying conditions compared to those with higher equivalent weights and lower acidities. The membrane electrode assembly was fabricated using a synthesized sulfonimide copolymer. The H-2/O-2 fuel cell performance was tested in a single cell, which revealed potential applicability of the sulfonimide membranes to polymer electrolyte fuel cells.