Poly(phenylene ether)s containing pendant perfluoroalkyl sulfonic acids with ion exchange capacity (IEC) of 1.17-1.83 mequiv/g were synthesized by the aromatic nucleophilic substitution reaction of a perfluoromonomer (decafluorobiphenyl or hexafluorobenzene) with 2,5-bis(4'-iodophenyl)hydroquinone, followed by the Ullman coupling reaction with potassium 1,1,2,2-tetraoro-2-(1,1,2,2-tetrafluoro-2-iodoethoxy) ethanesulfonate. The membranes prepared by solution casting showed high oxidative stability against hot Fenton's reagent. Even in the hydrated state, water uptake of the membranes was significantly low (19.3-28.0 wt %) due to the high fluorine content. High proton conductivity (similar to 1.0 x 10(-1) S/cm) was observed in all membranes under high relative humidity (95% RH). Furthermore, the membrane with IEC = 1.83 mequiv/g still maintained high proton conductivity of 3.3 x 10(-3) S/cm even under low relative humidity (30% RH), which was comparable to that of the Nafion 117 membrane (4.1 x 10(-3) S/cm). This proton conductivity was the highest data among the aromatic polymers containing pendant perfluoroalkyl sulfonic acids. Atomic force microscopy observation supported the formation of the phase-separated structure that tie hydrophilic domains were well dispersed and connected to each other.