Combining the information obtained at small and larger overpotentials and comparing with theoretical microkinetic rate expressions, our analysis suggests a potential-dependent change in the HOR/HER mechanism, and a concomitant change in the H 2 reaction order, going beyond current mechanistic understanding. In the potential range around equilibrium, our results are in agreement with a Tafel-Volmer mechanism with a rate-determining Volmer step, as previously proposed by Rheinländer et al. At increased overpotentials in HOR direction, however, a transition in the mechanism to Heyrovsky-Volmer with a rate-determining Heyrovsky step is suggested. This would reconcile the conclusions by Rheinländer et al. and Sheng et al. on the debated reaction mechanism of the HOR/HER in alkaline conditions. While we wish to emphasize that alternative mechanistic interpre-tations of our results at hand remain possible, we already note that an augmented data set including pH variations will be presented in a subsequent publication providing definite mechanistic conclusions.