The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) crystal surfaces were investigated by surface vibrational spectroscopy via sum frequency generation (SFG) both under vacuum and high pressure conditions with 10 Torr cyclohexene and various hydrogen pressures from 30 up to similar to 600 Torr. At high pressures, the gas composition and turnover rate (TOR) were measured by gas chromatography. In vacuum, cyclohexene on Pt(111) undergoes a change from pi/sigma-bonded, sigma-bonded cyclohexene and C-C6H9 surface species to adsorbed benzene when the surface was heated from 130 to 330 K. A site-blocking effect was observed at saturation coverage of cyclohexene that caused dehydrogenation to shift to somewhat higher surface temperature. At high pressures, however, none of the species observed in vacuum conditions were detectable. 1,4-cyclohexadiene (1,4-CHD) was found to be the major species on the surface at 295 K, even with the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300 and 400 K with 1,3-cyclohexadiene (1,3-CHD) on the surface, as revealed by SFG. Further increasing the surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate and both 1,3-CHD and 1,4-CHD were present on the surface simultaneously. The simultaneous observation of the reaction kinetic data and the chemical nature of surface species allows us to postulate a reaction mechanism at high pressures : cyclohexene hydrogenates to cyclohexane via a 1,3-CHD intermediate and dehydrogenates to benzene through both 1,4-CHD and 1,3-CHD intermediates. Isomerisation of the 1,4-CHD and 1,3-CHD surface species is negligible.