An empirical equation [E = E(0) - blogi - Ri- m exp (ni)] was shown to fit the experimental cell potential (E) vs. current density (i) data for proton exchange membrane fuel cells (PEMFCs), at several temperatures, pressures, and oxygen compositions in the cathode gas mixture. The exponential term compensates for the mass-transport regions of the E vs. i plot; i.e., the increase in slope of the pseudolinear region and the subsequent rapid fall-off of the cell potential with increasing current density. As has been previously shown, the terms E(0) and b yield the electrode kinetic parameters for oxygen reduction in the PEMFC and R represents the resistance, predominantly ohmic and, to a small extent, the charge-transfer resistance of the electro-oxidation of hydrogen. The exponential term characterizes the mass-transport region of the E vs. i plot. The parameter n has more pronounced effects than the parameter m in this region. A physicochemical interpretation of these parameters is needed.