A series of sterically encumbered, sulfonated, alternating poly(arylene ether) copolymers were synthesized so as to compare the effects of incorporating ketone and sulfone groups on the proton conductivity and performance of fuel cells of membranes prepared. Therefore, the polymers were prepared by polymerization of 4,4'-difluorobenzophenone or bis(4-fluorophenyl)sulfone with a novel monomer, 2'',3'',5'',6''-tetraphenyl-[1,1':4',1'':4'',1''':4''',1''''-quinquephenyl]-4,4''''-diol. Subsequent sulfonation and solution casting provided membranes possessing ion exchange capacities ranging from 2.1 to 2.8 mmol g(-1). The water uptake of sulfonated polymers increased with increasing IEC, ranging from 68 to 146 % at 80 A degrees C in water compared with 30 % for N117. Proton conductivities ranged between 3 and 17 mS cm(-1) at 30 A degrees C/40 % RH, and 88 and 263 mS cm(-1) at 80 A degrees C/95 % RH compared with 10 and 100 mS cm(-1) for N117, respectively. The proton mobilities ranged between 76.6 x 10(5) cm(2) V-1 S-1 and 170.4 x 10(5) cm(2) V-1 S-1 at 80 A degrees C, greater than that of N117. No substantial difference is observed between membrane prepared from polymers incorporating ketone and analogous membranes incorporating sulfone groups, thus inferring that ketone and sulfone groups in the polymer main chain do not appear to have any discernible difference upon membrane properties.