This paper presents a preparation route which allows the formation of a poly(phenylene) ionomer containing merely sulfone units (-SO2-) connecting the phenyl rings and in which each phenyl ring is monosulfonated (100% degree of sulfonation). This corresponds to an ion exchange capacity (IEC) of 4.5 mequiv g(-1) (equivalent weight of EW = 220 g equiv(-1)). The preparation succeeded in a two-step process comprising a polycondensation reaction of sulfonated difluorodiphenyl sulfone with sodium sulfide, yielding sulfonated poly(phenylene sulfide sulfone), and the subsequent oxidation to the corresponding sulfonated poly(phenylene sulfone). The polymer was characterized by elemental analysis, NMR, IR, GPC, viscosity measurement, TGA in air and in pure water vapor atmosphere, DSC at low and high temperatures, and ac impedance spectroscopy. Room temperature water absorption isotherms have been determined by equilibrating samples at different relative humidities. Under the chosen reaction conditions, polymers with molecular weights up to M-W approximate to 61 000 g mol(-1) were obtained, corresponding to intrinsic viscosities up to 0.73 dL g(-1). The water-soluble ionomer exhibits a very high density (rho = 1.75 g cm(-3) in the dry form), no glass transition or melting temperature, and a very high thermooxidative and hydrothermal stability. The latter is attributed to the specific molecular structure consisting of extremely electron-deficient aromatic rings. At high temperature (T = 110-160 degrees C) and low relative humidities (rh = 50-15%) the proton conductivity exceeds that of Nafion by a factor of 5-7.