In this work, we discuss the effect of multiphase flow dynamics on the performance of a PEM electrolyzer. We obtained images of a flow system consisting of O-2 and water at two stages of gas production: gas evolution via bubbles and gas exiting through the channels of a flow field. We processed the obtained images of bubble evolution with a MATLAB-based bubble detection and counting algorithm, and we found that the bubble detachment sizes remained invariant within a water flow range between 0.07 and 4.65 1 h(-1). We measured an average bubble detachment radius of 22.47 mu m. We applied a bubble force balance developed by van Helden et al. [1] to model the observed effect of water flow on bubble evolution, and we found that the bubble detachment radius is a weak function of water flow when the water flow is below 60 1 h(-1). We found that the variables that affect the bubble detachment radius the strongest were the electrode's hydrophobicity and pore size. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.