The H/D exchange between ethylene and hydrogen (or deuterium) on Pt(111) surfaces was monitored by laser-induced thermal desorption in combination with Fourier-transform mass spectrometry (LITD/FTMS) and by reflection-absorption infrared spectroscopy (RAIRS). In the case of coadsorbed C2D4 + H-2 the LITD experiments show that the H/D exchange reaction takes place at temperatures as low as 215 K, well below those needed for the decomposition of ethylene to ethylidyne. It was also found that exchange on adsorbed C2D4 is significantly easier with H-2 than viith C2H4, indicating that this H/D substitution requires surface hydrogen but not the dissociation of ethylene, and suggesting that ethyl moieties are the most probable intermediates for the reaction. It was also determined that the dosing order of the reactants influences the kinetics of the reaction, which means that specific surface ensembles are required in the exchange; both the rate and the degree of H/D substitution in ethylene are increased by dosing hydrogen (deuterium) beforehand. In addition to the LITD experiments, the rate of formation of ethylidyne that takes place above 250 K was also monitored with RAIRS. It was shown that, when coadsorbed with deuterium, the extent of deuterium substitution in the resulting ethylidyne is largely determined by H/D exchange between the original ethylene molecules and surface hydrogen, and that direct H/D exchange between ethylidyne and surface hydrogen is measurable only at low ethylidyne coverages and higher temperatures. Finally, it was established that the rate of ethylidyne formation does not depend on the coverage of hydrogen on the surface, a result that points to an ethylidene intermediate in that reaction. The data are discussed on the basis of a mechanism in which the H/D exchange and ethylidyne formation are parallel processes which involve different surface species.