A series of new acid monomers with different acid groups based on sulfonimide were synthesized. Acidity of the sulfonimide monomers could be controlled by changing substituted groups on the sulfonimides. Copolymerization of the acid monomers was carried out with acrylonitrile, and the proton exchange membranes were cast from solutions of the resulting copolymers. The membranes were mechanically strong, flexible, and transparent. Adsorption isotherm behavior for water uptake by the membranes was found to depend on the acidity of the starting monomers and hydrophobicity and hydrophilicity of the copolymers. The membranes exhibited high proton conductivities under humidifying conditions, which depended fairly on temperature as well as the water uptake. At a constant relative humidity, membranes of the strong acids showed conductivities higher than those of the weak acids.