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The reaction of NH 3 + O 3 on the singlet potential energy surface has been investigated theoretically using CCSD(T) and G3B3 methods with the 6-311++G(3df,3pd) basis set based on the geometric parameters optimised at the B3LYP/6-311++G(3df,3pd) level of theory. The calculated results revealed that the reactants are firstly associated with the adduct NH 3 -O 3 , through a barrier-less process and two stable collision complexes, C1 and C2 have been considered. Six kinds of products P 1 (HOO + H 2 NO), P 2 (OH + H 2 NO 2 ), P 3 (O 2 + H 3 NO), P 4 (HNO + H 2 O 2 ), P 5 (HNO 2 + H 2 O) and P 6 (NO + HO 2 + H 2 ) are obtained through a variety of transformations of C1 and C2. Thermodynamic parameters of all possible products show five production channels which are spontaneous. After taking into account the reaction barrier and enthalpy, the most probable reaction pathways are P1(2) and P4. Also, P 4 is the most stable product in comparison with the others and P 1 yields the other important products from the kinetic viewpoint; this is relatively stable, with a negative Gibbs free energy. Branching ratio results show P 1 is the major product at low temperature. Finally, the rates constant have been computed only for the P1(2) and P4 pathways by related theories in the temperature range of 200 -2500 K.

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