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 induced semi-crystallinity in the polymer, as confirmed by molecular simulations and wide angle X-ray diffraction patters. The semi-crystalline SN polymer membranes exhibited excellent chemical and mechanical properties, better than those of their phenylene counterpart (SP). In particular, the water uptake and swelling ration 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(sq) s(-1)) compared to Nafion 212(330 x 10(-8) cm(sq) s(-1)) at 30 degrees Celsius in 10 M methanol. Moreover, sulfide- and naphthalene-based chemical structure and semi-crystalline nature of hte SN membranes enhanced their DMFC single cell performance and long-term stability during fuel cell operation. ; peer reviewed: yes ; NRC Pub: yes