The hydrogenation of ethylene on Pt(111) single-crystal surfaces was studied by combining measurements of the kinetics of reaction using mass spectrometry detection with the simultaneous characterization of the species present on the surface using reflection− absorption infrared spectroscopy. The kinetics measured by us matches past reports on the same system, with zero-and first-order dependence on the partial pressures of ethylene and hydrogen, respectively, and extensive H−D exchange if D 2 is used instead of H 2 . The reaction takes place in the presence of an alkylidyne surface layer, which forms immediately upon exposure of the clean surface to the reaction mixture and can be removed by hydrogen or another olefin but at rates 1−2 orders of magnitude slower than the ethylene-toethane conversion. The nature of the alkylidyne surface species changes slightly upon being exposed to high pressures of hydrogen, with the carbon in the terminal methyl moiety acquiring some sp 2 character. Moreover, the alkylidyne hydrogenation rate shows an inverse relationship with H 2 pressure and is reduced by the presence of olefins in the gas phase. Turnover frequencies for the olefin hydrogenation reaction under pressures in the Torr range are high, as reported repeatedly in the past, but the corresponding reaction probabilities are quite low, below the 10 −4 range. In contrast, almost unit reaction probability was observed here in effusive collimated molecular beam experiments emulating intermediate pressure conditions.