Nafion and poly(phenyl sulfone) are simultaneously electrospun into a dual-fiber mat. Follow-on processing of the mat produces two distinct membrane structures: (1) Nafion reinforced by a poly(phenyl sulfone) nanofiber network and (2) Nafion nanofibers embedded in inert/uncharged poly (phenyl sulfone) nanofiber network. For structure 1, the Nafion component of the fiber mat is allowed to soften and flow to fill the PPSU interfiber void space without damaging the PPSU fiber structure (by use of a mat compression step followed by thermal annealing). For structure 2, the PPSU material in the mat is allowed to soften and flow into the void space between Nafion nanofibers without damaging the Nafion fiber structure (by mat compression, exposure to chloroform solvent vapor, and then thermal annealing). Both membrane structures exhibit similar volumetric/gravimetric water swelling and proton conductivity, where the conductivity scales linearly with Nafion volume fraction and the swelling is less than expected based on the relative amounts of Nafion. The in-plane liquid water swelling of membranes with Nafion reinforced by a poly(phenyl sulfone) nanofiber network is always less than that of the inverse structure. On the other hand, the mechanical properties of membranes with Nafion nanofibers embedded in poly(phenyl sulfone) are superior to membranes with the opposite structure. Compared to other fuel cell membranes, the nanofiber composite membranes exhibit very low in-plane water swelling and better mechanical properties, which translates into improved membrane/MEA longevity in a hydrogen/air open circuit voltage humidity cycling durability test with no loss in power production as compared to a Nafion 212 membrane.