The oxygen reduction reaction (ORR) kinetics of a high-surface-area carbon-supported platinum catalyst (Pt/C) were measured in an operating proton exchange membrane fuel cell (PEMFC). The ORR kinetics of Pt/C can be described over a wide range of temperature, pressure, and current density using four catalyst-specific parameters: transfer coefficient, exchange current density, reaction order with respect to oxygen partial pressure, and activation energy. These parameters were extracted using a combined kinetic and thermodynamic model, either referenced to the reversible cell potential (i.e., using exchange current density as activity parameter) or referenced to a constant ohmic-resistance-corrected (i.e., iR-free) cell voltage. The latter has the advantage of using an activity parameter (activity at 0.9 V iR-free cell voltage) which can be measured explicitly without extrapolation, in contrast to the exchange current density required in the former model. It was found that much of the variation in the published values for these catalyst-specific kinetic parameters derives from applying the same parameter name (e.g., activation energy) without specifying which of its many possible definitions is being used. The obviously significant numerical differences both for "oxygen reaction order" and for "activation energy" due to different definitions (often tacitly assumed and rarely explicitly stated in the literature) are illustrated by the kinetic ORR parameters which we determined for Pt/C: (i) at zero overpotential, where reaction order and activation energy are similar to 0.5 and 67 kJ/mol, respectively, and (ii) at 0.9 V iR-free cell voltage, where reaction order and activation energy are similar to 0.75 and 10 kJ/mol, respectively. (c) 2006 The Electrochemical Society.