Polymer electrolyte fuel cells (PEFCs) are expected to play a pivotal role in heavy-duty transportation, but careful benchmarking of the kinetics of the O₂-reduction reaction (ORR) and mass transport properties dictating their performance is needed to render them market-competitive. This assessment generally relies on the extrapolation of the kinetic behavior observed at low current densities to the high currents at which mass transport limitations become significant. However, this kinetic regime is generally assumed to obey a simple Tafel law that does not consider the impact of the relative humidity (RH) on the availability of ORR-active sites. To shed light on the implications of this simplified approach, in this study, we compare the ORR parameters and mass transport losses derived from it with those computed using a more complex kinetic model that incorporates site-availability effects. Our results provide an original insight on the impact of the relative humidity on the ORR-performance of catalysts with different active site distributions and show that the mass transport resistances derived by these two quantification procedures are essentially identical as long as the slope of the kinetic Tafel line is adjusted according to the RH.