The thermal chemistry of trideuterioethylene on Pt(lll) surfaces was characterized by reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The vibrational data indicate that thermal activation of a saturated layer of adsorbed ethylene by heating the surface to 350 K produces surface ethylidyne species with an isotopic composition of about 45% perdeuterioethylidyne and 55% dideuterioethylidyne. The hydrogen TPD data confirm that result, and also highlight the fact that the proportion of hydrogen-to-deuterium elimination from the original ethylene molecules changes with initial coverage. To explain these observations a previously proposed three-step mechanism is used where ethylene first isomerizes to ethylidene and ethylidene then either isomerizes back to ethylene or dehydrogenates td ethylidyne. The changes in hydrogen isotope removal probability with coverage are explained by changes in the relative rates between the latter two steps because of the fact that the outgoing hydrogen in the dehydrogenation step requires an empty surface site that may be Mocked by ethylene molecules at high coverages.