Theoretical investigations are carried out on the reactions F + CHBrF2 -> CBrF2 + HF (R1) and Br + CHBrF2 -> CBrF2 + HBr (R2) by means of direct dynamics method. The minimum energy path (MEP) is obtained at the BH&H-LYP/6-311G(d,p) level, and energetic information is further refined at the QCISD(T)/6-311+G(2df,2p) (single-point) level. The rate constants for both reactions are calculated by canonical variational transition state theory (CVT) with the small-curvature tunneling (SCT) correction in a wide temperature range 200-3000 K. The theoretical rate constant is in good agreement with the available experimental data. Furthermore, the rate constants of reaction Cl + CHBrF2 CBrF2 + HCl (R3) calculated in the other paper are added to discuss the effects of halogen substitution on the rate constants of this class of hydrogen abstraction reactions. (c) 2006 Published by Elsevier B.V.