Electrocatalysis in the cathodic H-2 evolution reaction (HER) is characterized by a dependence of exchange current-densities, i(o), on metal properties represented by "volcano relations", e.g. between log i(o) and chemisorption energy of H. A maximum in log i(o) arises with the Pt metals that has been rationalized by the standard Gibbs energy, DeltaG(H)(o), of chemisorption of the H intermediate from 1/2H(2) being zero, corresponding to half coverage by H on Pt or other electrode metal sites. However, at the Pt metals, their surfaces are already virtually fully covered at the reversible potential by underpotential deposited (UPD) H so the H intermediate is not directly chemisorbed at free sites on the Pt metal surface but in some less strongly bound way on, or amongst, the already UPD H film which modifies the electronic properties of the Pt surface for H-2 evolution. The consequences of this situation for interpretation of volcano relations for the HER are examined as is also the sequence of experimental apparent enthalpies of activation for the HER at various metals and alloys.