Abstract For the first time, pore network modelling is applied to polymer electrolyte membrane (PEM) electrolyzers. Realistic sintered titanium powder-based porous transport layers (PTLs) are generated via stochastic modelling, and we determine the influence of PTL-catalyst coated membrane (CCM) contact, pore and throat sizes, and porosity on two-phase transport. The PTL-CCM interfaces exhibit large open pore spaces, which are catastrophic for effective reactant transport due to the localized preferential accumulation of oxygen gas. As expected, increasing the sizes of the pores and throats results in higher oxygen gas saturations; however, we concurrently observe a surprisingly dominant mass transport effect whereby liquid water permeability is enhanced. The numerically generated PTLs are further tailored for enhanced mass transport by imposing a porosity gradient. By increasing the porosity from the PTL-CCM interface to the PTL-flow field interface, lower gas saturations and higher liquid water permeabilities are observed. With the opposite porosity gradient, the majority of pores adjacent to the high porosity CCM region become invaded with oxygen gas. We recommend a backing layer (microporous layer) at the CCM-PTL interface that improves the contact and permeation of product oxygen away from the CCM, thereby enhancing the performance of the PEM electrolyzer.