The reactions of adsorbed propylene on Pt(lll) in the presence and absence of hydrogen have been studied over 13 orders of magnitude in pressure using infrared-visible sum frequency generation. In the presence of near ambient pressures of hydrogen and propylene at 295 K, propylene hydrogenation was observed to proceed from a physisorbed intermediate (pi-bonded propylene) through a 2-propyl species (Pt-CH(CH3)(2)) to propane, Under UHV conditions in the absence of hydrogen, di-sigma-bonded propylene was found to dehydrogenate to propylidyne just below room temperature and then to vinylmethylidyne (M=CCHCH2) at 450 K on the May to graphite formation.