Vanadium diffusion through Nafion membranes was investigated under conditions applicable to vanadium redox flow batteries (VRFBs). In this case, we examine the effects of interdiffusion of VO2+ with protons, V3+ and VO2+ by a 'cross-diffusion' experiment in which VO2+ and the other ion diffuse from opposite sides of the membrane. Vanadium concentrations in reservoirs designed to flow solution over both sides of the membrane were monitored using UV/vis and EPR spectroscopies. The permeability of each ion was determined and diffusion coefficients determined from permeability and measurements of ion uptake by the membrane. The VO2+ permeability is highest when H+ is the only ion on the counter side, and decreases in the following order: H+ > VO2+ > V3+. (C) 2017 The Electrochemical Society. All rights reserved.