This investigation examines characteristics of sulfonated polyether imides (SPEI) with various ion exchange capacity values (EEC) and completes previous work to enable its blends to be adopted as polyelectrolyte in direct methanol fuel cells (DMFC). Polyether imides (PEI) were sulfonated by using chlorosulfonic acid as the sulfonating agent and chloroform as the solvent. The structure of SPEI was observed by FTIR and H-1 NW. The sulfonate or sulfonic acid content of the polymers, expressed as a number per repeat unit of the polymer, was accurately determined by elemental analysis and conductometric titration. Physical properties such as solubility, intrinsic viscosities, thermal stability, and glass transition temperature (T-g) were studied for both PEI and SPET. TGA-FTIR verified that sulfonic groups, attached to the aromatic ring in the PEI backbone, are split at 230-350 degrees C, but the main-chain splitting temperature of SPEI is similar to that of pure polymer. The sulfonated samples exhibited good solubilities and increased glass transition temperatures (T(g)values) as degree of sulfonation (DS) increased; two T-g values were detected when IEC was sufficiently high. (c) 2007 Wiley Periodicals, Inc.