A technique to measure an average mass-transport resistance of a polymer electrolyte fuel cell in situ is described. Experimental polarization curve data are extrapolated to the limiting current, at which the driving force for diffusion can be accurately estimated. The technique gives very reproducible results and is sensitive to changes in the mass-transport characteristics of the cell. Using a resistance-in-series approach, the mass-transport characteristics of the electrode and gas diffusion media (GDM) can be separated. For the particular system used in this study, it is estimated that the GDM accounts for about 25% of the total mass-transport resistance of the fuel cell assembly. Bulk gas diffusion accounts for less than half of the measured mass-transport resistance. The measured mass-transport parameter may also be utilized for comparison of fuel cell systems, analysis of the effect of operating conditions, or for tracking changes during durability testing.