The development of proton conducting membranes based on poly(styrene sulfonic acid-co-1-vinylimidazole-co-styrene) (PSSA-co-PVIm-co-PS)/poly(vinyl chloride) (PVC) blends is firstly reported. PSSA-co-PVIm-co-PS with three different terpolymer compositions were synthesized via conventional free radical polymerization by varying styrene feed. Successful syntheses were confirmed by H-1-nuclear magnetic resonance spectroscopy (H-1-NMR), elemental analysis, and Fourier transform infrared spectroscopy (FTIR). Hydrolytically stable PSSA-co-PVIm-co-PS/PVC blend membranes were prepared via solution-cast method. Scanning electron microscopy (SEM) images along with two glass transition temperatures observed from differential scanning calorimetry (DSC) suggested immiscible polymer blends. Water uptake and ion exchange capacity (IEC) were found to decrease with increasing PS content in terpolymer. All blend membranes had high thermal decomposition onsets of 230 A degrees C. The blends demonstrated high storage moduli at room temperature and high oxidative stability. Proton conductivity at 25 A degrees C of membranes equilibrated with water vapor was found to depend on PS content, and a maximum conductivity of 7.8 x 10(-5) S/cm was achieved from 1:1:4/PVC blend membrane. For dry membranes, the effect of PS amount on proton conduction was not clearly observed at elevated temperatures (100-120 A degrees C).