The effect of the surface structure of Pt (111) and Pt (100) has been investigated for cyclohexene hydrogenation and dehydrogenation, and ethylene hydrogenation by using sum frequency generation. Cyclohexene dehydrogenation is a structure sensitive reaction, and the rate was found to proceed more rapidly on the Pt (100) crystal surface than on the Pt (111) crystal surface. On Pt (100), the major reaction intermediate during cyclohexene dehydrogenation was 1,3-cyclohexadiene, whereas on Pt(111), both 1,3- and 1,4-cyclohexadiene were present. Both 1,3- and 1,4-cyclohexadiene can dehydrogenate to form benzene, although the reaction proceeds more rapidly through the 1,3-cyclohexadiene intermediate. Because of this, the structure sensitivity of cyclohexene dehydrogenation is explained by noting that there are both a fast and slow reaction pathway for Pt (1), whereas there is only a fast reaction pathway on Pt (100). Ethylene hydrogenation is a structure insensitive reaction. Both ethylidyne and di-sigma -bonded ethylene are present in both Pt (111) and Pt (100) under reaction conditions, although the ratio of the concentrations of the two species are different. The rate of the reaction was found to be 11 +/- 1 and 12 +/- 1 molecules per site per second for Pt (111) and Pt (100), respectively. Since the reaction rate is essentially the same on the two surfaces, while the concentration of ethylidyne and di-sigma -bonded ethylene are different, these species must not be the active species which turnover under catalytic ethylene hydrogenation. The most likely species which turnover are pi -bonded ethylene and ethyl, and their concentrations are near the detection limit of SFG.