Chorine- and fluorine-containing methyl radicals have been investigated by ab initio methods. Geometries and vibrational frequencies were derived with quadratic configuration methods at the QCISD/6-311G(d,p) level of theory, and energies via QCISD(T)/6-311+G(3df,2p) and Gaussian 3 theory. Anharmonicity of the out of plane bending mode was taken into account by numerical integration of the Schrodinger equation with a potential derived from a relaxed scan of this mode. The results are in good accord with experimental data where available. For the radicals CHF2, CF3, CH2Cl, CHCl2, and CCl3, we compute Delta(f)H(298)(0) values of -241.2, -465.9, 117.0, 91.1, and 72.2 kJ mol(-1), respectively, which agree with well-established experimental values to within 2.2 kJ mol(-1). For the more poorly characterized molecules CH2F, CHClF, CClF2, and CCl2F we compute Delta(f)H(298)(0) values of -29.0, -63.8, -274.7, and -94.3 kJ mol(-1), respectively, with recommended confidence limits of +/-4.1 kJ mol(-1). (C) 2003 American Institute of Physics.