Creep experiments utilizing a membrane inflation geometry were used to characterize the mechanical properties of pentablock copolymer membranes designed for water purification applications. Membranes were formed by casting from micellar solutions, and structure development during the casting process was followed in real time with grazing incidence small angle X-ray scattering. The thin film morphologies were consistent with previously published results obtained for much thicker solvent cast films. The biaxial creep compliance of the films for applied stresses in the megapascal range increased substantially as the degree of sulfonation and consequent hydration of the middle block was increased. The sulfonation level was much more important than the overall degree of ordering in determining the creep compliance of the hydrated membranes. (C) 2014 Wiley Periodicals, Inc.