Sulfonated poly(arylene sulfide sulfone nitrile)s (SN) were synthesized to investigate the effects of naphthalene units in the polymer backbone on membrane properties. The small and planar naphthalene in the main chain reduced interdomain distance, as confirmed by molecular simulations and small-angle X-ray scattering patterns. The SN polymer membranes exhibited excellent chemical and mechanical properties, better than those of their phenylene counterpart (SP). In particular, the water uptake and swelling ratio of the SN membranes were much lower than those of the SP membranes. Furthermore, the SN membranes exhibited a greatly reduced methanol permeability ((9-17) x 10(-8) cm(2) s(-1)) compared to Nafion 212 (330 x 10(-8) cm(2) s(-1)) at 30 degrees C in 10 M methanol. Moreover, sulfide- and naphthalene-based chemical structure of the SN membranes enhanced their DMFC single cell performance and long-term stability during fuel cell operation.