A family of multiblock copolymers consisting of alternating fully sulfonated hydrophilic poly(phenylene sulfone) and hydrophobic poly(phenylene ether sulfone) segments are prepared and characterized. The multiblock copolymers are formed by the coupling of preformed hydrophilic and hydrophobic blocks using a specially designed coupling agent. The block lengths (degree of polymerization) of both segment types were varied in order to control the ion exchange capacity. Solution cast films show spontaneous nanophase separation leading to distinct bicontinuous morphologies with correlation lengths around 15 nm. The hydrophobic phase gives the membranes their advantageous viscoelastic properties even at high temperatures under both wet and dry conditions, while proton conductivity takes place within the hydrophilic phase. Since the properties of fully sulfonated poly (phenylene sulfone)s are locally preserved within the hydrophilic domain, the membranes show very high proton conductivity and high hydrolytic stability. The very high degree of water dispersion within the hydrophilic domains leads to very low electro-osmotic water drag. Because of their superior transport and stability properties these multiblock copolymers have a great potential for use as a substitute for perfluorosulfonic acid membranes which are used as separator materials in electrochemical applications such as polymer electrolyte membrane (PEM) fuel cells and redox flow batteries.