AbstractMany modern catalyst materials exploit a strained surface layer as the active component. Here, we explore how the catalytic activity is affected by changes in the lattice parameter, focusing on the hydrogen evolution reaction on Au and Pt electrodes in H2SO4 as a model process. We present a lock-in technique that allows the modulation of the reaction current to be followed in situ while a small cyclic elastic strain is imposed on the electrode material. We find that tensile strain enhances the exchange current density and the reactivity at low overpotential, ΔE, whereas the trend is inverted and the reactivity diminished at higher ΔE. We introduce kinetic rate equations for Heyrowsky and Tafel kinetics, allowing for strain dependence of the hydrogen adsorption enthalpy as well as the activation enthalpy. The results link the reactivity modulation to electrocapillary coupling coefficients that are open to investigation by experiment or ab initio computation. The inversion in sign of the coupling as the function of ΔE emerges in agreement with experiment.