The reaction of CH3 and CD3 radicals with hydrogen and deuterium have been investigated, acetone being used as a source of methyl radicals. The results indicate (1) that the substitution of D2 for H2 has a considerable effect, (2) the substitution of CD3 for CH3 has relatively little effect. (3) It is concluded that, contrary to the results of Burton et al., the activation energy of the reaction[Formula: see text]is 9.7 ± 0.6 kcal.