We studied the adsorption structures and reactions of C-6 hydrocarbon molecules (cyclohexene, cyclohexane, 1-methylcyclohexene, n-hexane, 2-and 3-methylpentanes, and 1-hexene) on Pt(111) using sum frequency generation (SFG) vibrational spectroscopy. The experiments were performed in the presence and absence of excess hydrogen and as a function of temperature. Upon cyclohexene adsorption on Pt(111) at 1.5 Torr, 1,3- and 1,4-cyclohexadienes and pi-allyl C6H9 were observed in the presence of excess hydrogen. Cyclohexane adsorption at 1.5 Torr resulted in the formation of cyclohexyl on the surface in the presence of excess hydrogen but pi-allyl C6H9 in absence of excess hydrogen. 1-Methylcyclohexene formed methylcyclohexenyl on the surface only in the presence of excess hydrogen. n-Hexane and 3-methylpentane adsorbed molecularly on Pt(111) at 296 K in the presence of excess hydrogen. 2-Methylpentane and 1-hexene were readily dehydrogenated to form metallacyclobutane and hexylidyne even at 296 K, regardless of the presence of excess hydrogen. n-Hexane was dehydrogenated to form hexylidyne or metallacyclic species at high temperature in the presence of excess hydrogen. Hexylidyne and metallacyclic species were also main surface intermediates in dehydrogenation of 2- and 3-methylpentane. The absence of excess hydrogen induced dehydrocyclization of n-hexane to form pi-allyl c-C6H9. On the basis of the SFG results, the mechanism of the n-hexane conversion to benzene is discussed.