The direct hydrogen abstraction mechanisms on (3)A" potential surfaces for the reactions of NH((3)Sigma(-)) with CH4, CH3F, CH2F2, and CHF3 have been studied systematically using ab initio molecular orbital theory. The G2(MP2) calculations reveal that all reactions involve significant energy barriers. The effect of fluorine substitution was examined. The NH + CHF3 reaction was found to possess the highest barrier among the four reactions. The barriers for both NH + CH3F and NH + CH2F2 reactions are about 2 kcal/mol lower than that for the NK + CH4 reaction. The rate constants for the four reactions have been deduced using transition-state theory with asymmetric Eckart tunneling correction and hindered rotor approximation over the temperature range 200-3000 K. The following least-squares-fitted expressions for the rate constants were obtained: k(1)(H)(NH+CH4) = (9.41 x 10(-18))T-2.28 e(-10233/T), k(2)(H)(NH+CH3F) = (1.69 x 10(-18))T-2.31 e(-9217/T), k(3)(H)(NH+CH2-F-2) = (1.52 x 10(-18))T-2.32 e(-9080/T), k(4)(H)(NH+CHF3) = (2.12 x 10(-18))T-2.29 e(-10750/T), in cm(3) molecule(-1) s(-1). The deuterium kinetic isotope effects have also been investigated. All reactions show the significant and "normal" kinetic isotope effects.