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 K. The following least-squares fitted expressions are recommended for practical applications: k1 = 1.72 × 10-17T1.73 e-28454/T, k-1 = 6.02 × 10-1.63T1.63 e630/T, k2b = 3.92 × 10-23T3.41 e-11301/T, k-2b = 11.8 × 10-23T3.02 e2487/T, in cm3/(molecule·s). The apparent activation energies calculated variationally for 300 ≤ T ≤ 1000 K, 58.3 and 25.6 kcal/mol for (1) and (2), respectively, agree well with experiments.