The atmospheric reaction between NO and ozone has been investigated using ab initio methods. The structures of all reactants, products, intermediates, and transition states of reaction 1 have been optimized and characterized at the UMP2(full) level of theory. The 6-31G(d), 6-311G(d), and 6-311G(df) basis sets have also been used to calibrate the effect of the basis set functions on the optimized structures and energies of all stationary points. Finally, we have reoptimized at the UMP4(SDQ, full)/6-31G(d) and 6-311G(d) levels. The energetics of the reaction has been studied more accurately within the G2 and G2(MP2) schemes. Also, QCISD(T)/6-311 G(d) single-point calculations have been performed by using UMP2(full)/6-31G(d), 6-311G(d), and 6-311G(df) geometries. The mechanism study of the NO + O-3 --> NO2 + O-2 reaction has shown that the transition between reactants and products is not direct, but it proceeds along two transition states (TS1 and TS2) separated by the intermediate (A). There is also the possibility of two different conformational (cis/trans) channels on the PES. In addition to the title reaction (the only one found under atmospheric conditions), we have also extended our QCISD(T)/6-311G(d)//UMP2(full)/6-31G(d) study to the reactions that might occur under high temperature or pressure conditions, such as NO + O-3 --> NO2 + O-2((1)Delta(g)), NO + O-3 --> ONOO + O(P-3), ONOO --> NO + O-2, ONOO --> NO2 + O(P-3), and ONOO --> NO3.