The morphology of a series of poly(t-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-t-butylstyrene) (tBS-HI-SS-HI-tBS) pentablock copolymer membranes with a range of ion exchange capacity (IEC) was investigated with small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Comparing the sizes of the micelle cores (2R), obtained from SAXS of the micellar solutions in cyclohexane/heptane, to the interparticle distances (d), obtained from SAXS and TEM imaging of the membranes, reveals that membranes with a low SS volume fraction (0.4, 0.7 and 1.0 mequiv./g IECs) show discrete SS microdomains within a HI-tBS matrix, while high SS volume fraction membranes (1.5 and 2.0 mequiv./g IECs) exhibit a bicontinuous microphase separated morphology without long-range order. As ion exchange capacity increases from 1.0 to 1.5 mequiv./g, the morphological transition from discrete SS microdomains to interconnected SS microdomains results in a significant increase in water vapor transport rate. When exposed to liquid water, the extent of water uptake and the increase in primary spacing of the membranes are greater at higher sulfonation levels suggesting that absorbed water plasticizes the hydrophilic SS microdomains. (C) 2012 Elsevier B.V. All rights reserved.