Understanding the mechanism of electrocatalytic reactions is challenging, in part, because relevant processes occur on timescales ranging from femtoseconds to tens of milliseconds. In principle potential perturbation experiments, in which the bias of a working electrode is altered for only femtoseconds and the resulting charge transfer or evolution of interfacial structure probed, are a way of separately interrogating the fastest relevant timescales, in particular the elementary step of charge transfer across the interface. In his paper recently published in ChemElectrochem Gregor demonstrates that it is possible to perturb the potential of single crystal Pt electrodes using femtosecond optical pulses and directly measure the resulting photocurrents. This new observable implies that, at least on Pt at potentials of hydrogen adsorption and evolution ease of charge transfer correlates with particular, goldilocks, structures of interfacial electrolyte and that interestingly the trend in reactivity of Pt single crystal surfaces with respect to H2 formation appears to correlate with ease of partial charge transfer across the Pt-H bond.