Reflection Absorption Infrared Spectroscopy (RAIRS) has been employed in a study of carbon monoxide, hydrogen and ethylene adsorption on a Pt(111) single crystal model catalyst. The experiments have been performed on apparatus consisting of a UHV system with isolable high pressure cell, the latter being interfaced to a Fourier Transform Infrared Spectrometer. In situ RAIRS studies have yielded activation energies for the formation and subsequent decomposition of the ethylidyne species on Pt(111). The kinetic parameters have been found to compare favourably with those previously determined by less direct methods. Rates of hydrogenation of the ethylidyne adlayer have been monitored in situ as a function of hydrogen pressure up to 1 atm. At pressures above approximately 1 mbar there is evidence for the population of an 'on top' form of adsorbed hydrogen, the presence of which brings about the onset of ethylidyne hydrogenation.