The reactions of thermal H and D atoms with C2H4 and C2D4 have been studied in helium diluent at room temperature with a discharge flow system time of flight mass spectrometric apparatus. New absolute measurements of the apparent bimolecular rate constants, under atom excess conditions, for the reactions D+C2H4, H+C2D4, and D+C2D4 have been obtained as a function of pressure. Mechanistic studies have also been carried out for the D+C2H4 and H+C2D4 reactions in the pressure region of 1–5 torr. The observed pressure dependence of the rate constants and the mechanisms for the two mixed cases have been found to differ markedly. The variation in over-all mechanism for the mixed isotope cases, ethyl-d1 and ethyl-d4, and the nonequilibrium kinetic isotope effects for chemically activated ethyl radicals are presented and compared with other work. RRKM theoretical calculations are shown to be consistent with the experimental observations for the four systems.