The all-iron flow battery is currently being developed for grid scale energy storage. As with all flow batteries, the membrane in these systems must meet stringent demands for ionic conductivity while limiting unwanted reactant (Fe3+) crossover. In addition, for the all-iron chemistry proton transport across the membrane is highly desirable to maintain the pH levels in the negative and positive electrolytes. Two membranes are considered, Nafion and an in-house developed composite based on Daramic (a commercial microporous separator) and poly(vinyl alcohol). Their performance is compared for various metrics specific to the all-iron flow battery chemistry.