Disulfonated poly(arylene ether sulfone) copolymer membranes are of interest for water purification by desalination. These negatively charged copolymers exhibit lower fouling and greatly improved resistance to oxidants such as chlorinated disinfectants compared to state-of-the-art highly cross-linked aromatic polyamide, porous polysulfone supported thin film composite (TFC) systems. A systematic series of controlled molecular weight 4,4'-biphenol and bisphenol A based partially disulfonated poly(arylene ether sulfone)s were synthesized with terminal amine functionalities via end-capping with m-aminophenol. The stoichiometric balance of the end-capping reagent, bisphenols, and two activated dihalides controlled M-n and degree of disulfonation. Oligomers with controlled molecular weights and ionic content were thermally cross-linked with a multifunctional epoxy resin (TGBAM) derived from methylene dianiline. The residual masses from boiling solvent extraction confirmed high gel fractions. The networks had improved salt rejection compared to linear controls due to reduced swelling, and this proved to be a valuable parameter for enhancing transport properties. The cross-linked highly sulfonated copolymers produced the best water purification properties to date observed for disulfonated polysulfone membranes by retaining high salt rejection with enhanced water permeabilities. For example, an epoxy-cross-linked 4,4'-biphenol-based 60% disulfonated polysulfone with an ion exchange capacity (IEC) of 1.85 mequiv/g had a salt rejection of 96.7% and a relatively high hydraulic water permeability of 1.18 (L mu m m(-2) h(-1) bar(-1)), compared to a linear 4,4'-biphenol-based 40% disulfonated polysulfone with a similar ionic content (IEC = 1.78) that only had a salt rejection of 92.5% and a hydraulic water permeability of 0.62 (L mu m m(-2) h(-1) bar(-1)).