The H 2 reaction order of the HOR, m HOR , was determined according to Eq. ( ) from the four kinetic current data points under different hydrogen partial pressures for each value of the potential, E, on the fixed potential scale. The resulting curve of m HOR as function of potential is plotted in Fig. . The reaction order has a relatively stable value of about m HOR ≈ 0.9, corresponding to an almost linear dependence of the HOR rate on the H 2 concentration. This is a very intuitive result given that hydrogen is the reactant for the HOR. Moreover, it justifies the assumption of first-order kinetics underlying the KL masstransport correction used in the treatment of the RDE data. In Fig. , the kinetic currents are re-plotted on the commonly employed overpotential scale. The corresponding reaction order, γ HOR , defined under constant overpotential (cf. Eq (8)) is plotted as a function of the overpotential in Fig. . Again, a relatively constant value is obtained, however, at about γ HOR ≈ 0.6. The much lower value of γ HOR in comparison to m HOR is a direct consequence of the H 2 -pressure-dependent relative shift of the kinetic current curves according to the Nernst equation ( ), when going from the fixed potential scale in Fig. to the overpotential scale in Fig. , which effectively moves the curves closer together. In fact, using the calculus of partial derivatives (applied to Eqs. ( ), (8), and ( ), together with Eqs. ( ) and ( )), it is straightforward to derive the following relationship between the reaction orders m HOR and γ HOR ,