Hydrogen abstraction by trifluoromethyl radicals from ammonia and ammonia-d3 has been studied in the temperature range 113–240°C. The Arrhenius parameters, based on Ayscough's value of 1013.36 cm3 mole–1 sec–1 for the combination of trifluoromethyl radicals, are: [graphic omitted] Comparison of the relative reactivities of methyl and trifluoromethyl radicals in their attack on ammonia and ammonia-d3 has been made; apparently both polar character and bond energy considerations have to be invoked to account for the observed enhanced reactivity of CF3 over CH3 radical attack. The isotope effect for the N—H bond has been measured and is in agreement with zero-point energy differences for the stretching of one N—H bond.