The role of the ionomer in catalyst layers is critical to the performance of PEM fuel cells. Attention needs to be paid not only to the inherent properties of the ionomer but also to the choice of dispersing solvent and the catalyst support, as these also control the porosity and proton conductivity of the catalyst layer in conjunction with the ionomer. The ionomer also influences electrochemical FC kinetics through its influence on surface adsorption and gas permeability. Mass transport of oxygen in cathode catalyst layers (CCLs) is of particular importance in achieving high current densities in proton exchange membrane fuel cells. The technical push toward low platinum loading in CCLs has resulted in a disproportionate transport resistance attributed to oxygen transiting through thin ionomer films to reach active platinum sites. The replacement of PFSA ionomer in the catalyst layer with hydrocarbon ionomers is thus particularly challenging as this often decreases electrochemical fuel cell kinetics and mass transport. For these reasons, permeability phenomena of oxygen at the ionomer/platinum interface has gained renewed interest. Electrochemical techniques, such as potential step chronoamperometry at microelectrodes, will be shown to be a useful to probe oxygen diffusion and oxygen solubility at catalyst/membrane interface. Data obtained under different conditions and with different ionomers is useful for understanding oxygen transport resistance through ionomer films in the context of low platinum loaded cathode catalyst layers