The Koutecky-Levich equation is based on the assumption of first-order kinetics, i.e., the kinetic rate is assumed to be linearly dependent on the concentration of the masstransport limited reactant. However, for an electrocatalytic multi-step reaction, it is not at all obvious that the first-order assumption is fulfilled a priori. Nevertheless, most RDE studies of the HOR have applied the Koutecky-Levich correction in a pragmatic way, taking for granted its validity and thus implicitly assuming such first-order kinetics. However, performing RDE experiments for the HOR/HER under different hydrogen partial pressures, Rheinländer et al. determined a reaction order of essentially 1/2 and developed a modified Koutecky-Levich mass-transport correction to account for this. Based on the reaction order of 1/2 and the results from Butler-Volmer fits of their kinetic current curves, they proposed a mechanism for the HOR/HER consisting of a fast Tafel step,