Fluorine-and chlorine-substituted methyl and methylene radicals have been studied by ab initio quantum chemical methods in order to determine the molecular structure and vibrational frequencies as well as the enthalpies of formation. The equilibrium geometries of radicals have been optimized at various levels, and the vibrational frequencies have been calculated at the optimized geometries. The calculated results, particularly at the MP2/6-311G(d) level, are in good agreement with the experimental data reported previously, suggesting that the present levels of theory can yield reasonably accurate estimates for the molecular properties of halogenated radicals when the experimental data are lacking. The standard enthalpies of formation at 298 K, Delta H-f degrees(298 K), of these radicals have been obtained, utilizing the electronic energies calculated by the G2 or the CBS method. Enthalpies of formation for methyl radicals calculated with the original G2 method are close to the experimental values although the claimed accuracy of 2 kcal/mol is not quite achieved, but the errors in enthalpies of formation from other methods are ra?her high. For methylene radicals whose enthalpies of formation show large discrepancies among the values reported, some assessments have been made based on the calculated enthalpies of formation.