Gas evolving electrodes are found in many electrochemical systems, such as water electrolyzers, where the dynamics of multiphase flow at these electrodes can strongly impact cell performance. This is especially true for membraneless water electrolyzers that have emerged in recent years as a promising approach to low cost hydrogen production. However, experimental and modeling efforts to characterize the multiphase dynamics in these systems can be non-trivial due to the complexity of the coupled chemistry and physics that underlie their operation. In this study, we utilize a high speed video (HSV) camera as a non-invasive analytical tool to better understand bubble dynamics in membraneless electrolyzers and to better quantify the void fraction of gas bubbles in the region directly downstream of the mesh flow-through electrodes. By detecting and quantifying the void fraction of bubbles immediately downstream of an operating electrode, the local current density distribution can be determined along the length of the electrode. This HSV-measured void fraction distribution is in good agreement with the modeled primary current distribution. This study also highlights the ability to utilize in situ HSV analysis to monitor gas evolution efficiency yields and bubble size distributions under varying operating conditions. (c) The Author(s) 2019. Published by ECS.