A three-dimensional (3D), two-phase numerical model was developed and presented as a useful tool for investigating oxygen bubble propagation in porous transport layers (PTLs) (otherwise known as gas diffusion layers (GDLs)) of polymer electrolyte membrane (PEM) electrolyzers. The volume-of-fluid (VoF) technique was employed to simulate the liquid-gas interface movement through liquid-saturated porous media designed to be representative of PEM electrolyzer PTLs. The circulation of the liquid within the channel and the porous domain was included in the model. Bubble propagation patterns and bulk saturations for porous material representations of commonly used PTLs were determined as a function of time leading up to the moment of breakthrough. Previously conducted experimental microfluidic investigations were used for model validation, and it was found that the numerical results were in good agreement with the numerical predictions. The validated model was used to calculate pressure variations in bubbles during propagation, and the highest threshold capillary pressure corresponding to a critical throat was introduced as a means to measure the efficacy of oxygen bubble removal. The information obtained from the developed numerical tool can be used for designing and evaluating PTL microstructures for next generation electrolyzer materials. (C) 2016 The Electrochemical Society. All rights reserved.