Poly(arylene ether sulfone)s (SPEs) block copolymers containing fluorenyl groups were synthesized for fuel cell applications. Post-sulfonation of the polymers by chlorosulfonic acid gave sulfonated block copolymers having highly sulfonated hydrophilic block. SPE block copolymer membrane with the ion exchange capacity (IEC) = 2.20 meq/g showed proton conductivity, 0.14 S/cm (80% RH) and 0.02 S/cm (40% RH) at 80°C, which is higher or comparable to that of perfluorinated ionomer (Nafion®) membrane. STEM observation of Ag-stained membranes revealed that phase separation of the SPE block copolymers was more developed than that of the SPE random copolymers. Long-term hydrolytic stability of these membranes has been confirmed.