The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C6H10, 200-600 Torr H-2, and 273-650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H-2 (H-2: C6H10 ratio = 20:60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (< 423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (> 573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.