Polymer electrolyte membrane (PEM) fuel cells are a promising means of reducing anthropogenic greenhouse gas emissions. However, mass transport losses hinder the performance of PEM fuel cells, impeding their commercialization. This thesis comprises of two studies that determine the impact of liquid water within the substrate on the oxygen transport behaviour of the fuel cell. First, the impact of channel and land region saturation on the oxygen transport properties of the substrate were determined. The oxygen transport properties of the substrate were severely affected by channel region saturation, while land region saturation had a relatively minor impact. Next, the contribution of the substrate to the oxygen transport resistance of a PEM fuel cell was determined. It was determined that significant oxygen transport resistance arises from the catalyst layer (CL) or CL-microporous layer interface. This thesis offers insight into designing next-generation components for improved PEM fuel cell performance.