Previous literature shows that electrocatalysis in the cathodic H-2 evolution reaction (HER) can be comparatively characterized at various metals by a 'volcano curve' relating the log of the exchange current-density to the bond energy of H chemisorbed to the metal or the standard Gibbs energy of chemisorption of H at the metal. The platinum metals lie at or near the apex of such a curve and this apex arises in the theoretical rationalization of the volcano plot when the standard Gibbs energy of dissociative chemisorption of H to the metal from H-2 is zero [R. Parsons, Trans. Faraday Sec. 34 (1958) 1053], whereas the corresponding bond energy to H from the gas-phase, or in underpotential deposition of H, is experimentally similar or equal to 250 kJ mol(-1), or equivalently, from 1/2H(2), - 35 kJ mol(-1), calculated as the enthalpy of chemisorption. However, at the platinum metals, uniquely, cathodic H, evolution takes place on a surface almost filled by underpotentially deposited H already at the H, reversible potential, so the binding energy of the H intermediate in the HER is not that to a free Pt surface. The consequences of this situation, in relation to the significance of the volcano curve for the HER, are discussed in detail with regard to differences between underpotential and overpotential deposited states of H at Pt.