The hydrogen abstraction reactions of atomic O (P-3) with CH3Cl and CH2Cl2 have been studied theoretically using ab initio molecular orbital theory for the first time. In the C-s symmetry, both reactions proceed over two potential-energy surfaces, (3)A(') and (3)A(') generated by the pseudo-Jahn-Teller effect. Two nearly degenerate transition states of (3)A(') and (3)A(') symmetries have been located for each hydrogen abstraction reaction from the C-H bonds. Geometries of the reactants, transition states, and products have been optimized at the second-order Moller-Plesset (MP2) level with the 6-311G(2d,p) basis set. The single-point energy calculations have been carried at the [QCISD(T)]/6-311+G(3df,2p) level. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths are discussed and compared. The total thermal rate constants were obtained from the sum of the calculated rate constants for the two surfaces using canonical variational transition-state theory (CVT) with small curvature tunneling effect (SCT). The results show that the variational effect is small and in the lower-temperature range, the small curvature tunneling contribution is important for each reaction. The calculated CVT/SCT rate constants show reasonable agreement with the recent experimental values, with a more pronounced curvature in the Arrhenius plot than in the experimental data. (C) 2003 American Institute of Physics.