A series of fully aromatic polymers having only sulfone bridges linking the aromatic rings have been synthesized via polycondensations and studied as proton-exchange membranes. Mixtures of tetrasulfonated 4,4′-bis[(4-chlorophenyl)sulfonyl]-1,1′-biphenyl (BCPSBP), non-sulfonated BCPSBP, and 4,4′-thiobisbenzenethiol were copolymerized by nucleophilic aromatic substitution reactions to obtain sulfonated poly(arylene thioether sulfone)s (SPATSs) with ion exchange capacities (IECs) between 2.0 and 4.0 mequiv g–1. The thioether bridges of the SPATSs were quantitatively oxidized to sulfone bridges to obtain the corresponding sulfonated poly(arylene sulfone)s (SPASs). Small-angle X-ray scattering of dry SPATS and SPAS membranes showed that the tetrasulfonated segments promoted a distinct phase separation of the ionic groups already at quite low ionic contents. The SPAS polymers degraded between 300 and 340 °C in air, which was significantly above the degradation temperature of the corresponding SPATSs polymers. Moreover, SPAS membranes showed a significantly lower water uptake than the corresponding SPATS membranes. SPATS and SPAS membranes with IEC values of 2.4 and 2.2 mequiv g–1, respectively, maintained high proton conductivity at low relative humidity (RH). At 30% RH and 80 °C, these membranes reached 8 and 10 mS cm–1, respectively. The latter value coincided with that recorded for the state-of-the-art perfluorinated NRE212 membrane under the same conditions. Thus, the SPAS materials combine a straightforward synthetic pathway with a very robust polymer structure giving high proton conductivity at reduced RH.