Sum frequency generation (SFG) surface vibrational spectroscopy has been used to identify reactive surface intermediates in situ during catalytic dehydrogenation reactions of high-pressure cyclohexane (C6H12) on the Pt(l 11) crystal surface in the presence and absence of high-pressure hydrogen. These experiments provide the first spectroscopic evidence of cyclohexyl (C6H11) as a reactive surface intermediate during the cyclohexane catalytic conversion to benzene at high pressure in the presence of excess hydrogen. In addition, it was proposed from temperature-dependent SFG experiments that dehydrogenation of cyclohexyl is a rate-limiting step in the cyclohexane catalytic conversion to benzene.