Nafion((R)) perfluorosulfonic acid (PFSA) and poly(vinylidene fluoride) (PVDF) were electrospun simultaneously as a polymer solution mixture using a single needle spinneret. The nanofiber morphology was highly unusual, with bundled 2-5 nmfibril strands of Nafion and PVDF aligned along the fiber axis. Membranes were made from fiber mats by a simple hot-pressing step, followed by thermal annealing, where the fibril morphology of Nafion and PVDF in the original nanofibers was retained in the membrane. The PVDF component in the final membrane served three roles, as an electrospinning carrier polymer for PFSA, a mechanical reinforcement, and a hydrophobic (uncharged) component to limit PFSA ionomer swelling. A series of single-fiber membranes with Nafion/ PVDF contents ranging from 40/60 to 90/10 wt%/wt% were prepared, characterized, and evaluated for use in a regenerative hydrogen/bromine fuel cell. As expected, there was a decrease in proton conductivity, water/electrolyte swelling, and Br-2/Br-3-permeability with increasing PVDF content. Membrane conductivity was lower than expected based on the weight fraction of PVDF, due presumably to an unusually large fraction of highly structured water. Nevertheless, a single-fiber Nafion/PVDF membrane with a thickness of 18 mu m and an 80/20 wt%/wt% Nafion/PVDF composition performed well in a H-2/Br-2 regenerative fuel cell due to a combination of low area-specific resistance and low Br-2/Br-3-crossover. Thus, with an electrolyte containing 0.9 M Br-2 in 1.0 M HBr, the maximum power output was 46% higher than that with a Nafion 212 membrane (1.31 vs. 0.90 W/cm(2)).