The reactions of O3 with HO2 and HNO have been studied at the G2M//BH&HLYP/6-311++G(2df,2p) level of theory. Both reactions can occur by H-abstraction by O3 and O-abstraction by HXO (X = O and N). The barriers for H-abstraction reactions were found to be lower and take place via the O3⋯HXO van der Waals complexes with 2.5 kcal/mol stability for O3⋯HO2 and 1.3 kcal/mol for O3⋯HNO. The rate constants predicted for the H-abstraction processes in the temperature range 150-1000 K given in cm3 molecule-1 s-1 can be represented by k1 = 6.34 × 10-30 T5.37 exp(885/T) for the former and k2 = 3.36 × 10-24 T3.59 exp(397/T) for the latter reaction, which is 3-5 times faster than the former between 150 and 500 K. The predicted rate constant and product branching ratio for the HO2 + O3 reaction are in close agreement with experimental data. The heat of formation of HO3 was estimated to be 7.4 ± 1.0 kcal/mol, HO3 was predicted to fragment with 0.7 kcal/mol barrier.