The HOR in mildly alkaline electrolyte proceeds through the alkaline path, limited by the mass transport of proton acceptors until sufficiently positive potentials where the acidic path dominates, limited by the mass transport of H2. In strong alkaline electrolytes, the HOR always proceeds through the alkaline path, limited by the mass transport of H2. Regarding the HER, in an alkaline electrolyte it is not associated with a limiting current since water serves as the proton donor. In contrast, the HER in acidic electrolyte is limited by the mass transport of proton donors until the potential becomes sufficiently negative, and the alkaline path dominates. While we varied the pH in the measurement series shown in Fig. and 6b and kept the partial pressure of molecular hydrogen constant to inter alia determine the reaction order with respect to the proton donor and acceptor species, in Fig. we show exemplary how the reaction order with respect to molecular hydrogen was determined. Inspired by Rheinländer et. al. , we varied the molecular hydrogen pressure at constant pH. But instead of using predefined gas mixtures of hydrogen in argon in separate experiments, we varied the argon and hydrogen gas supply during one measurement series and used the OCP value of the HOR/HER to determine the bulk activity of molecular hydrogen. Calculating the molecular hydrogen concentration from the H2 limiting current yielded comparable values at intermediate pH but deviated significantly at high pH, which is attributed to the decreased solubility of hydrogen at high pH values. With this qualitative description in mind, we can identify three characteristic regimes of the HOR/HER i-V curves: