The hydrogen evolution (HER) and the hydrogen oxidation reaction (HOR) were studied on platinum single crystals in a sulfuric acid solution over the temperature range 274-333 K. We found, for the first rime, that at a fixed temperature (274 K) the exchange current densities (i(o)) increase in the order (111) much less than (100) < (110), with the i(o) on the (110) surface being 3 times that on the (111) surface. We also found that each crystal face has an unique, temperature-dependent Tafel slope for the HOR, and that the activation energies for the HER and the HOR decrease in the sequence Delta H-111(#) > Delta H-100(#) > Delta H-110(#), the same sequence as the order these differences in activation energy with crystal face are attributed to structure-sensitive heats of adsorption of the active intermediate, H-ad, whose physical state is unclear. We analyzed the kinetic data with a model for the coupling of this unknown state, Had, with the well-known adsorbed state of hydrogen, H-upd, whose adsorption energy is strongly structure-sensitive. We concluded that on Pt(110), the reaction follows the Tafel-Volmer mechanism with the Tafel (recombination) step rate determining. On Pt(100), the reaction follows the Heyrovsky-Volmer sequence, with the Heyrovsky (ion-atom) reaction step being the rate-determining step. The reaction mechanism on Pt(111) could not, however, be resolved by analyzing the kinetic parameters. The relatively low activity and high activation energy for the (111) surface is attributed to strong repulsive interaction between H-ad adatoms on this surface.