Ab initio calculations have been performed on hydrogen abstraction from nine halogen-substituted methanes (F, Cl) by chlorine atom attack. The characteristics (geometry, energy, frequencies) of the transition states have been determined by the GAUSSIAN program with the 6-31G(**) basis set. MP2 and MP4 energies have been corrected in different ways. The BAC-MP4 method of Melius leads to a good agreement between theory and experiment for thermodynamic and kinetic parameters of eight of the nine reactions studied here. In the case of the reaction involving the CHCl2F compound, no experimental value for the activation energy is available and the BAC-MP4 method leads to the value of 2.3 kcal mol(-1). We propose a method, based upon a combination of isodesmic processes and the intrinsic energy concept proposed by Marcus, that gives better energetics and is simpler to use. With this method, we find 3.3 kcal mol(-1) for the activation energy of reaction involving the CHCl2F compound. Attempted application of the MP-SAC2 method of Truhlar leads to values less close to experimental data. This is possibly due to the insufficient size of the atomic basis set used. It is shown that a quadratic correlation (of the Marcus type) between the activation barriers and the reaction enthalpies calculated by various methods is more suitable for analysis of the reactivity trends in the series of reactions of halomethanes with chlorine atoms than the linear correlation of Evans-Polanyi type. Moreover, we have shown that, for the studied series and whatever the method used, a correlation exists between the activation energies and the geometrical and electronic structure of the transition states which can explain the influence of the substituent effect on the reactivity of the halomethanes. The calculated preexponential factors of the usual transition state theory are almost the same in the series. Finally, the influence of tunneling on the rate constant values is shown to be important at room temperature.