Potential energy surfaces for the reactions of NH3 with NOx (x = 1, 2) have been studied by ab initio molecular orbital Gaussian 1 (G1) and Gaussian 2 (G2) methods. Both reactions have been shown to be endothermic and to proceed by the abstraction of a hydrogen atom from ammonia to produce NH2 and HNOx. The calculated heats of reaction are in close agreement with experimental measurements. Reaction 1, NH3 + NO, does not have a reverse barrier at the G2 level of theory. Reaction 2, NH3 + NO2, can occur by three channels, leading to HNO2 (2a), cis-HONO (2b), and trans-HONO (2c), and each mechanism involves the formation of NH2 . HNO2 or NH2 . HONO intermediate complexes. Mechanism 2b has been found to be dominant. Theoretical rate constants for (1), (2b), and their reverse reactions have been computed by VTST in conjunction with detailed balancing for the temperature range of 300-5000 R. The following least-squares fitted expressions are recommended for practical applications : k(1) = 1.72 x 10(-17)T(1.73) e(-28454/T), k(-1) = 6.02 x 10(-17)T(1.63) e(630/T), k(2b) = 3.92 x 10(-23)T(3.41) e(-11301/T), k(-2b) = 11.8 X 10(-23)T(3.02) e(2487/T), in cm(3)/(molecule . s). The apparent activation energies calculated variationally for 300 less than or equal to T less than or equal to 1000 K, 58.3 and 25.6 kcal/mol for (1) and (2), respectively, agree well with experiments.