Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton and deuteron transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures in proton and deuteron form respectively. Ion currents up to 1 A / cm² were obtained for proton transfer across graphene, and were diminished by a factor of 14 for otherwise identical cells pumping deuterium ions. Proton / deuterium pumping rates were nearly identical to each other for cells having no graphene. Arrhenius analysis of variable temperature studies of deuterium pumping across Nafion | graphene | Nafion membranes yielded an activation energy of approximately 0.5 +/- 0.1 eV for deuterium transfer across graphene.