The effects of gas diffusion layer (GDL) and electrode microstructure, which influence the catalyst layer and catalyst-membrane interface on the performance of a membrane electrode assembly (MEA) for gas-phase electrolysis and the separation of CO2 were experimentally characterized. Several types of GDL materials, with and without a microporous layer (MPL), were characterized using scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area analysis. The diffusion of reactants through the GDL materials was measured to determine the effects on the microstructure and chemical properties on mass transport. The effects on the GDL structure and chemistry were determined through evaluation of Pt-IrO2 MEAs with different GDL materials using constant-current measurements. Increasing the thickness of the MPL and hydrophobicity within the GDL assist with retaining water within the membrane and catalyst layers, which results in greater performance at high current densities.