where R is the gas constant, T the temperature, and F the Faraday constant, and E 1 bar RHE is the equilibrium potential at a standard reference pressure of p ⊖ = 1 bar. While experimental studies in electrocatalysis generally make use of the overpotential scale, the fundamental physico-electrochemical reaction conditions are determined on a fixed potential scale with an arbitrary, but fixed potential reference. The latter can be, e.g., the equilibrium potential E 1 bar RHE at a hydrogen partial pressure of 1 bar and a fixed electrolyte pH. It is on a fixed potential scale that the microscopic rate expressions of certain reaction mechanisms are formulated, from which the corresponding expressions on an overpotential scale are derived subsequently. Likewise, the Koutecky-Levich equation used for RDE mass-transport correction is derived using rate expressions on the fixed potential scale. As a consequence, when assessing the validity of the first-order assumption underlying Koutecky-Levich masstransport corrections, the value of the reaction order on a fixed potential scale is decisive, but has been largely overlooked in previous studies dealing with the H 2 -pressure dependency of the HOR/HER. It is therefore critical whether the respective reaction order is evaluated on an overpotential scale or using a fixed reference potential, not only for mass-transport correction, but also for kinetic analysis and mechanistic interpretation of the experimental data. In electrochemistry, two distinct definitions of the reaction order exist,