The effect of the surface structure of Pt (1 1 1) and Pt (1 0 0) 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 (1 0 0) crystal surface than on the Pt (1 1 1) crystal surface. On Pt (1 0 0), the major reaction intermediate during cyclohexene dehydrogenation was 1,3-cyclohexadiene, whereas on Pt (1 1 1), 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 1 1), whereas there is only a fast reaction pathway on Pt (1 0 0). Ethylene hydrogenation is a structure insensitive reaction. Both ethylidyne and di-σ-bonded ethylene are present in both Pt (1 1 1) and Pt (1 0 0) 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 (1 1 1) and Pt (1 0 0), respectively. Since the reaction rate is essentially the same on the two surfaces, while the concentration of ethylidyne and di-σ-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 π-bonded ethylene and ethyl, and their concentrations are near the detection limit of SFG.