The hydrogen abstraction reaction of O(P-3) with CH3Br has been studied theoretically for the first time. Two nearly degenerate transition states of (3)A" and (3)A' symmetries have been located for this abstraction reaction. Geometries have been optimized at the UMP2 level with the 6-311 G(2d, p) basis set. The G2MP2 method has been used for the final single-point energy calculation. On the basis of the ab initio data, the rate constants have been deduced over a wide temperature range 200-3000 K using canonical variational transition-state theory (CVT) with a small curvature tunneling effect (SCT). The calculated CVT/SCT rate constants exhibit typical non-Arrhenius behavior, a three-parameter rate-temperature formula is fitted in units of cm(3) molecule(-1) s(-1) as follows: k(T) = (2.83 x 10(19))T-2.33 exp (-2115.97/T). (C) 2002 Published by Elsevier Science B.V.