The influence of a microporous layer (MPL) on polarization and electrochemical impedance behavior of proton exchange membrane fuel cells (PEMFCs) was investigated. Commercial carbon backing electrode with MPL applied on one of its sides were employed for both the anode and cathode. The ohmic, charge-transfer, and mass-transport resistances at various current densities were obtained by deconvolution of electrochemical impedance spectroscopy data. PEMFCs with an MPL showed higher performance and lower variability in the charge-transfer and mass-transport regions of the polarization curve (current density above 300 mA cm(-2)) within a batch of identically built cells. For cells with and without MPL, the charge-transfer resistance decreased while mass-transport resistance increased with an increase in current density. The difference in charge-transfer resistance for cells with and without MPLs was found to be statistically insignificant due to the large variability in data for cells without MPLs. Cells with MPLs demonstrated lower mass-transport resistance compared to cells without MPL. The time constant for the mass-transport process probed at a low-frequency regime of the impedance was obtained from the Warburg impedance. Among the various possible oxygen-transport processes, the estimated time constant for oxygen transport in the porous transport layer (PTL) was found to be within an order of magnitude of the Warburg-impedance-derived value. Accordingly, it was assessed that the presence of an MPL helped reduce the water saturation in the PTL, thereby improving the oxygen transport to the cathode catalyst layer.