with concentrations c Ox/Red and forward/backward rate constants k f/b (E) as a function of the electrode potential E on a fixed potential scale. Consequently, the fixed-scale reaction orders, m, defined analogous to Eq. ( ) but with c Ox (or c Red ) instead of p H2 , have the expected unity (or zero) values for the forward and backward reaction directions, respectively. Transforming to an overpotential scale, however, the corresponding reaction orders, γ, defined analogous to Eq. ( ), assume fractional values depending on the respective transfer coefficients, thus rendering the kinetic analysis more complicated. The same rationale applies to the elementary steps of electrocatalytic multistep reactions, such as the HER/HOR, for which reason kinetic analysis on a fixed potential scale is expected to provide clearer insight than the conventionally used overpotential scale. With this motivation, and inspired by the experimental approach of Rheinländer et al., in this work we performed RDE experiments of the HOR/HER on a polycrystalline platinum electrode in alkaline electrolyte saturated at various hydrogen partial pressures. Unlike previous analyses, we rigorously employed a fixed potential scale for determining reaction orders and performing Butler-Volmer fits. This has profound consequences for the assessment of self-consistency with respect to the mass-transport correction employed, as well as the mechanistic interpretation of the kinetic parameters obtained. Importantly, our kinetic analysis suggests a potential-dependent transition in the reaction mechanism of the