Proton-conducting hydrocarbon membranes often require an ion exchange capacity (IEC) in excess of 2 mmeq/g to achieve a conductivity sufficiently high for technological applications (typically > 50 mS/cm). Membranes with high IEC are often brittle when dry and swell excessively when fully hydrated. Increasing the crystallinity of hydrocarbon membranes is hypothesized to increase phase segregation thereby enhancing their mechanical integrity in water by reducing their propensity to swell excessively, and to provide an ionic network when water contents are reduced when RH < 100% and under elevated temperatures. In this work we examine the effect of increasing the crystallinity of highly sulfonated graft copolymer membranes by blending with polycrystalline poly(vinylidene difluoride) (PVDF). The addtion of PVDF increases the crystallinity, reduces the ion exchange capacity, reduces swelling, yet does not deleteriously affect proton conductivity. The extent to which crystalinity increases and water swelling is reduced was found dependent on the molecular weight of the PVDF. The incorporation of high m. wt. PVDF enhanced crystallinity to a lesser degree than incorporating low m. wt. PVDF but swelling was reduced to a greater extent. This observation is explained by an increase in elastic forces of the membrane that oppose osmotic swelling due to an increase in physical connectivity of the PVDF crystallites via long PVDF polymer chains that span multiple crystallites. The addition of PVDF did not compromise proton mobility in the ionic phase and as a consequence of the much higher proton concentrations attained, proton conductivities were significantly enhanced. Finally, the properties of membranes containing different ratios of ionomer and high m. wt. PVDF are described. Optimal compositions of these components are found in membranes which possess minimal swelling in water and have proton conductivity that is less susceptible to large fluctuations in RH. (C) 2014 Elsevier B.V. All rights reserved.