Branched/crosslinked sulfonated polyimide membranes incorporating superior mechanical properties, high proton conductivity, and excellent fuel cell performance were successfully developed. The resulting polymer electrolytes displayed conductivity values of about 0.2 S (.) cm(-1) at 120 degrees C and 100% relative humidity. In a single H-2/O-2 fuel cell system at 90 degrees C, they exhibited reasonably high fuel cell performances comparable to that of Nafion 112.