Main-chain-type sulfonated polyimides (SPIs) from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-bis(4-aminophenoxy) biphenyl-3.3'-disulfonic acid (BAPBDS) and 4,4'-bis (4-aminophenoxy)biphenyl (BAPB) were used as electrolyte membranes for polymer electrolyte fuel cells (PEFCs). The SPI membranes swelled anisotropically in water by four to five times more largely in thickness direction than in plane direction of membrane. The cells with the SPI membranes showed excellent polarization performances, which were a little better than that for Nafion 112, i.e., using highly humidified H-2 and O-2 gases with a pressure of 0.3 MPa, the cell voltages with current densities of 0.5 and 1 A/cm(2) were 0.76 and 0.69 V, respectively, at 90 degrees C. The polarization performance was rather weakly dependent on the cell temperature and on the humidifier temperature. The cells showed good short-tenn stability up to 50 h. The proton conductivities in thickness direction of membrane were calculated from the membrane resistances evaluated from the complex impedance spectra of the operating cells. They were 0.24 and 0. 16 S/cm for the homo- and co-SPI membranes (2/1 in molar ratio of BABPDS/BAPB), respectively. The SPI membranes were rather isotropic in relation to proton conductivity in spite of their anisotropic morphology. (c) 2004 Elsevier Ltd. All rights reserved.