Surface vibrational sum frequency generation (SFG) has been used to identify surface species in situ during catalytic reactions of 1.5 Torr cyclohexene (C6H10) in the presence and absence of 15 Torr H-2 on Pt(111) crystal surfaces. Various surface species were observed during the reactions in the range 303-483 K. Upon adsorption at 303 K in the absence of H-2, cyclohexene is dehydrogenated to form 1,4-cyclohexadiene (C6H8). At 323 K, this species converts to pi-allyl c-C6H9 intermediate. At temperature higher than 423 K, benzene coexists with C6H9 on the surface. Similar results were obtained in the presence of 15 Torr H-2 except that in this circumstance 1,3-cyclohexadiene coexists with 1,4-cyclohexadiene at 303 K, and with C6H9 at temperature higher than 403 K. This study reports, for the first time, the existence of pi-allyl c-C6H9 as a major surface species in the presence of high-pressure cyclohexene and H-2 on Pt(111). In addition, adsorbed 1,3-cyclohexadiene has been observed as a reactive intermediate in the process of dehydrogenation to benzene. Finally, it has been found that excess hydrogen increases the disordering and decreases the surface coverage of the C6H9 on the surface.