Through-plane in operando neutron radiography was used to differentiate between anode feed water and cathode water (transported through the membrane) to resolve oxygen gas bubbles in the anode. In this work, we investigated the impact of cathode purging on the electrochemical performance of polymer electrolyte membrane (PEM) electrolyzers as well as its impact on neutron radiographic imaging to inform our recommendations on how to prescribe an optimal cathode purging rate. We found that excessive dry nitrogen purging in the cathode led to an increase in ohmic resistance and a decrease in the open circuit voltage. Therefore, an optimal nitrogen purging rate must be prescribed during visualization studies to maximize the precision of oxygen gas quantification without unintentionally altering the electrochemical performance. We demonstrated that the optimal cathode nitrogen purge rate can be determined by monitoring the spatial distribution of the anode oxygen gas and that this optimal purge rate is a function of the operating current density. (c) 2018 Elsevier Ltd. All rights reserved.