A series of poly(phenylene)-based polyelectrolytes were synthesized from 1,4-bis(2,4,5-triphenylcyclopentadienone)benzene and 1,4-diethynylbenzene by Diels-Alder polymerization. Postsulfonation of this high molecular weight and thermochemically stable poly(phenylene) with chlorosulfonic acid resulted in homogeneous polyelectrolytes with controllable ion content (IEC = 0.98-2.2 mequiv/g). Fuel cell relevant properties such as high proton conductivity (123 mS/cm), chemical/thermal stability, and film toughness suggest that this polyelectrolyte material shows promise as a potential candidate for polymer electrolyte membrane fuel cells. Physical properties of this material, such as water uptake, thermal stability, and proton conductivity, are reported with respect to ion exchange capacity and compared to Nafion and a series of sulfonated poly(ether sulfone)s.