The interface between a membrane and electrode is a critical performance component for polymer electrolyte fuel cells (PEFCs) that to this point has gone largely unreported. This paper reports a method for quantifying interfacial resistance between a membrane and electrodes in polymer electrolyte membrane fuel cells. The interfacial resistances presented were obtained by subtracting electronic contributions from nonmembrane fuel cell resistance. Nonmembrane resistances have been determined from in situ fuel cell measurements of high-frequency resistance as a function of membrane thickness in direct methanol fuel cells. From these measurements, interfacial resistances were found to increase from a low of 8 m Omega cm(2) for commercial Nafion up to 57 m Omega cm(2) for recast Nafion membranes, both 1100 equivalent weight. Two aromatic hydrocarbon membranes were also tested and found to have interfacial resistances in between those of commercial and recast Nafion. The method by which electronic cell resistance has been estimated is presented. Membrane conductivity calculated from the study of interfacial resistance is presented as well, comparing fuel cell membrane conductivities to vapor and liquid equilibrated freestanding membranes. Finally, the impact of interfacial resistance on fuel cell performance is presented. (c) 2007 The Electrochemical Society.