The sulfuric acid, vanadyl (VO2+) and water equilibrium in Nafion membranes contacted by solutions containing these species is described. Of particular interest is the influence of composition on ionic transport behavior in membrane separators for an all-vanadium redox flow battery (VRFB). Ex-situ membrane conductivity measurements were conducted on Nafion 117 membranes equilibrated in electrolyte solutions of varying sulfuric acid and vanadyl ion concentrations. Electrolyte species imbibed in the membrane were analyzed by an experimental protocol including titration, ICP-OES and weight analysis. Sulfuric acid in the membrane can increase proton concentration but reduce proton mobility by reducing water content. In a mixed vanadyl/proton form Nafion, vanadyl has a mobility of 6.28 x 10(-5) cm(2) . V-1 . s(-1), much lower than proton mobility of 8.79 x 10(-4) cm(2) V-1 s(-1) in H+-form Nafion. The presence of vanadyl in Nafion can also decrease the proton mobility: u(H+) = (8.79 - 8.04 x x(VO2+)) x 10(-4)cm(2)V(-1)s(-1). With equilibration in a practical electrolyte containing 5 mol . dm(-3) total sulfate, Nafion's conductivity is decreased due to uptake of vanadyl ions. (C) 2013 The Electrochemical Society. All rights reserved.