The mechanism of the dehydrogenation of methylcyclohexane (MCH) on Pt(111) has been studied using HREELS, TPD, and BPTDS. Upon warming a monolayer of MCH dosed at 100 K,congruent to 55% desorbs and congruent to 45% dehydrogenates to a pi-allylic species at congruent to 240 K. This pi-allylic intermediate dehydrogenates further to a benzyl species at congruent to 310 K. Adsorbed benzyl is stable up to congruent to 450 K on Pt(111), but at higher temperatures further dehydrogenation occurs.