We demonstrate that small and narrow hydrophilic conducting morphology in sulfonated aromatic membranes leads to much better fuel cell performance at medium temperature and low humidity conditions than those with larger hydrophilic domains. A comparison of three types of sulfonated poly(arylene ether sulfone)s (SPAES) with random, block, and graft architecture indicates that small hydrophilic domain sizes (less than 5 nm) appear to be important in supporting water retention under low relative humidity (RH) conditions intended for medium temperature (less than 100 degrees Celsius) fuel cell applications. The graft copolymer outperformed both a random copolymer and multiblock copolymer at 120 degrees Celsius and 35% RH fuel cell operating conditions due to capillary condensation of water within the 3-5 nm hydrophilic domains. ; peer reviewed: yes ; NRC Pub: yes