It is well established that the transport of molecular hydrogen from the bulk of the solution to the electrode surface is limiting the hydrogen oxidation at sufficiently high overpotentials. This effect can be suppressed to some extent by either using an ultramicroelectrode or a rotating disc electrode (RDE). Both techniques lead to a controlled diffusion process, but the latter is preferred due to its versatility, higher surface area, and easier interpretation. An overview of the process on a RDE is displayed in Fig. . At positive overpotentials, the HOR current is higher than the HER current leading to a net consumption of molecular hydrogen on the surface and vice versa. At steady state the molecular flux of hydrogen to/from the electrode must be proportional to the spatial activity gradient within the diffusion layer (Fig. ), according to Fick's law. This gradient is not constant in the typical RDE system as implied by some publications , instead it is proportional to 𝑒 -𝑦 3 with 𝑦 being the distance from the electrode . The often-cited diffusion layer thickness is a mathematical construct as if the problem would have a constant gradient (Eq. 34a). This equivalence allows us to rewrite e.g. Eq. 32e and obtain an implicit expression for the HOR/HER current (Eq. 34b), including the current density dependent activity of molecular hydrogen at the surface. We chose to exemplify the mass transport effects on the H-V path with a Heyrovský rds but will generalize it in the following. With increasing positive overpotentials, the oxidation of molecular hydrogen (HOR) is inhibited by the reduced surface activity of molecular hydrogen. At negative overpotentials more hydrogen is produced than consumed at the electrode leading to a local increase of the hydrogen activity and a corresponding promotion of the HOR current that consequently reduces the net (negative) observed current (Fig. ).