The hydrogen evolution reaction (her) and the hydrogen oxidation reaction (hor) are studied on Pt(111), Pt(100), and Pt(110) single crystal surfaces in 0.1 M KOH over the temperature range 275-333 K. The results demonstrated that the kinetics of the her/hor are structure sensitive processes, with Pt(110) being about ten times more active than either of the atomically 'flatter' (100) or (111) faces at 275 K. At higher temperatures, however, the value of the exchange current density differs by less than a factor of two between Pt(110) and Pt(111). The difference in activity with crystal face is attributed to the structure sensitive adsorption of underpotentially deposited hydrogen (H-upd) and hydroxyl species (OHad) and the effect these species have on the formation of the electroactive intermediate, H-opd, whose physical state is uncertain. It is proposed that in the vicinity of the Nernst potential H-upd and OHad may have two modes of action on the kinetics of the her/hor: a blocking effect from competition for the same sites with molecular H-2 and H-opd, and an energetic effect altering the adsorption energy of the reactive intermediate. The significant differences of the her/hor kinetics in alkaline versus acid electrolyte are suggested to arise mainly due to the presence of OHad even close to the reversible potential of the her/hor in alkaline electrolytes.