Hydrogen-deuterium exchange in adsorbed methylidyne, CHa, on Pt{110}-(1x2) has been studied for the first time using supersonic D-2/H-2 molecular beams, which provides new insights into the reversible hydrogenation of adsorbed hydrocarbon fragments. The exchange reaction is extremely facile at surface temperatures of 350-450 K and proceeds via a Langmuir-Hinshelwood reaction between D-a and a CHa fragment to produce gas phase H-2 and HD. The CDa+H-a (i.e., reverse) reaction was also studied and was found to proceed more slowly. Both exchange reactions were successfully modeled and the difference in reaction rates is explained using zero point energy differences alone. Finally, we demonstrate that with high incident D-2 fluxes CHa can be completely deuterated to produce gaseous CHD3 and CD4.