Three Nafion (R) membranes of similar composition but different thicknesses were operated in a 3-cell 1 kW class all vanadium mixed acid redox flow battery. The influence of current density on the charge/discharge characteristics, coulombic and energy efficiency, capacity fade, operating temperature and pressure drop in the flow circuit will be discussed and correlated to the Nafion (R) membrane thick-ness. Material costs associated with the Nafion membranes, ease of handling the membranes, and performance impacts will also be discussed. (C) 2015 Elsevier B.V. All rights reserved.