The various criteria for the mechanism of the hydrogen electrode reaction are considered with particular reference to the pressure coefficient of the reaction rate. The techniques of studying electrode processes at pressures up to 1500 atm are described, and the volume of activation derived for the hydrogen evolution reaction has been found to be −3.4 ml mole−1. This negative value is not in accord with either of the two mechanisms usually proposed for this reaction. It suggests that the rate‐determining step is the stage i.e., the emission and hydration of metallic electrons.