A series of sulfonated homo- and random co-polyimides (co-SPI) based on 2,4-diaminobenzene-sulfonic acid (2,4-DABS) and 2,5-diaminobenzenesulfonic acid (2,5-DABS) has been synthesized via conventional two-step polyimidization method. 2,4-DABS and 2,5-DABS were used as sulfonated diamine compounds, 4,4'-oxydianiline (ODA) and 4,4'-diaminodiphenyl sulfone (DDS) were used as non-sulfonated diamine compounds. Mixtures of sulfonated and non-sulfonated diamine compounds were reacted with benzophenonetetracarboxylic dianhydride (BTDA) to obtain co-SPI membranes. Molar ratios of sulfonated to non-sulfonated diamine were systematically varied to produce copolymers of controlled compositions. The co-SPIs were evaluated for thermal oxidative stability, ion exchange capacity (IEC), water uptake, proton conductivity, solubility, and hydrolytic stability. Proton conductivity and hydrolytic stability of the co-SPIs were compared with the fully aromatic polyimide, homo-SPIs (BTDA/2,4-DABS and BTDA/2,5-DABS). Regarding thermogravimetric analysis (TGA) analysis, it is concluded that desulfonation temperature in the range of 200-350 degrees C suggests high stability of sulfonic acid groups. co-SPIs with 40 mol% of 2,4-DABS showed similar or higher proton conductivity than Nafion (R) 117 in water. Proton conductivity values of the co-SPIs were mainly a function of IEC and water uptake. Consequently, the optimum concentration of 2,4-DABS was found to be in the range of 30-40 mol% from the viewpoint of proton conductivity, IEC, and hydrolytic stability. Copyright (C) 2008 John Wiley & Sons, Ltd.