A series of aromatic sulfonated polyimides (SPIs) bearing sulfophenoxy side groups have been successfully synthesized and evaluated as polymer electrolyte membranes for fuel cell applications. The SPIs had high viscosity and gave tough and flexible membranes. The SPI membranes showed anisotropic membrane swelling in water with much larger dimensional change in thickness direction than in plane one. They showed the better proton-conducting performance even in the lower relative humidity (RH) range than the other SPI membranes, for example, a high proton conductivity of 0.05 S/cm at 50 % RH and 120 degrees C. They maintained high mechanical strength and conductivity after aging in water at 130 degrees C for 500 h, showing much better water stability compared with the main-chain-type SPI and side-chain-type SPI membranes reported so far. In polymer electrolyte fuel cells (PEFCs) operated at 90 degrees C and 84-30%RH, they showed fairly high cell performances and have high potential for PEFC applications. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1463-1477, 2009