The synthesis and characterization of a series of new poly(ethylene oxide) (PEO) soft segment-containing six-member ring sulfonated polyimide (SPI) copolymers are described in this paper. One-step high temperature polymerization method was used to prepare the SPI copolymers from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-diaminostilbene-2,2'-disulfonic acid (DSDSA), and diamine-terminated poly(ethylene oxide) (PEO-diamine, MW= 1000). The relative ratio of the sulfonic acid-containing hard segments to the PEO-containing soft segments was controlled through varying the molar ratio of DSDSA to PEO-diamine. Flexible, transparent, and mechanically strong free-standing membranes were successfully obtained. The membranes were characterized with ion-exchange capacity, Fourier transform infrared spectra, thermo-gravi metric analysis, water sorption, proton conductivity, and fuel cell performance measurements. The results showed that the new SPI membranes exhibited desirable mechanical properties and thermal stability as well as better proton conductivities than Nafion (R) 115 at high relative humidity (RH) levels (>50%) at both 70 and 120 degrees C. The results from the fuel cell performance measurements indicated that the SPI membrane containing 5 mol% (12.4 wt.%) PEO soft segments had similar fuel cell performance as Nallion degrees 112 at 70 degrees C and 80% RH, but better fuel cell performance than Nafion degrees 112 when the current density was higher than 0.32 A/cm(2) at 120 degrees C and 50% RH. (C) 2008 Elsevier B.V. All rights reserved.