The reactions of O-3 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 O-3 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 O-3 center dot center dot center dot HXO van der Waals complexes with 2.5 kcal/mol stability for O-3 center dot center dot center dot HO2 and 1.3 kcal/mol for O-3 center dot center dot center dot HNO. The rate constants predicted for the H-abstraction processes in the temperature range 150-1000 K given in cm(3) molecule(-1) s(-1) can be represented by k(1) = 6.34 x 10(-30) T-5.37 exp(885/T) for the former and k(2) = 3.36 x 10(-24) T-3.59 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 + O-3 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. (c) 2007 Elsevier B.V. All rights reserved.