The cost of the polymer electrolyte membrane (PEM) fuel cell must undergo significant reductions before the widespread adoption of PEM fuel cell powered automotive drivetrains can be achieved. Eliminating the need for active anode humidification is one strategy for reducing the cost and system size of the PEM fuel cell. In this study, we investigated the impact of anode gas inlet relative humidity (RH) on membrane hydration and the associated electrochemical performance of the PEM fuel cell. The anode gas inlet RH was varied to study the impact on fuel cell potential, during which simultaneous in operando visualizations were performed using synchrotron X-ray radiography, and electrochemical impedance spectroscopy was used to gain an understanding of the membrane hydration and water dynamics. The thickness of a Nafion (R) N115 membrane expanded by up to 26 mu m (20% of nominal thickness) compared to the manufacturer specification, as a result of changes in membrane hydration. Through this work, we present the utility of synchrotron X-ray radiography for tracking changes in membrane hydration of an operating PEM fuel cell. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.