A membrane inflation experiment was used to measure the mechanical response of ABCBA pentablock copolymer membranes designed to transport water through the coalescence of water-swollen polyelectrolyte domains within the polymer. Creep resistance of the block copolymers originates from glassy A blocks of either poly(p-methyl styrene) or poly(t-butyl styrene), and water permeability is attributed to the polystyrene C block that is partially sulfonated. The creep resistance of the membranes in the wet state was enhanced by replacing the poly (t-butyl styrene) A blocks with poly(p-methylstyrene), or by decreasing the molecular weight of the polyelectrolyte C block. Both of these substitutions increased the creep resistance of the membranes in a wet state while retaining a high degree of water permeability.