Deuterium was dissociated on a hot tungsten filament and the atom concentration measured by isothermal calorimetry. The recombination coefficient of deuterium atoms on a glass surface, coated with metaphosphoric acid, was found to be 3.8 × 10−5, and similar to that found for hydrogen atoms. The reactions of H-atoms and D-atoms with ethylene were found to be very rapid. The effects on the yields of the products and on their isotopic composition of variations of reactant flow rate, atom concentration, pressure, and atom-detector position were studied. The major products were methanes, ethanes, and ethylenes, with minor amounts of propanes and butanes. The methanes were always highly deuterated while the ethanes were slightly deuterated. A mechanism is proposed to explain the observations based on a flow pattern in the reaction zone. The possibility of differences in the reaction rates of variously deuterated intermediates is also discussed.