The geometries and standard enthalpies of formation (Delta(f)H degrees) of 4 paraffins and 31 alkyl radicals at 298 K were obtained by means of complete basis set 4 (CBS-4) ab initio molecular orbital theory. A comparison of the geometry of a molecule and the radicals derivable from it via elimination of a H atom from different positions suggests that only a negligible change occurs in the geometry of the groups immediately adjacent to the radical center. The CBS-LF Delta(f)H degrees s scaled by an empirical correction display an average deviation of 0.4 kcal mol(-1) from the experimental data. Group values (GVs) were evaluated from the corrected ab initio Delta(f)H degrees s by simultaneous parameter estimation. The group additivity (GA) rule was checked by a statistical analysis of the theoretical data. The derived GVs describe the corrected ab initio Delta(f)H degrees s with an average deviation of less than 0.3 kcal mol(-1), which indicates that the GA rule is applicable to alkyl radicals, and the Delta(f)H degrees s of alkyl radicals can be predicted with an accuracy similar to that attained with existing experimental methods. New values of radical, radical-adjacent, and (C)(2)-C-(C)over dot(C) gauche GVs are suggested; together with the accepted alkane GVs, these yield accurate Delta(f)H degrees s of alkyl radicals within estimated errors of 0.5 kcal mol(-1). Application of the GVs derived from small species is predicted to perform less satisfactorily in the estimation of Delta(f)H degrees s of C-4-C-6 species: the average deviation increases from 0.3 to 0.6 kcal mol(-1), while the correlation of the individual deviations and carbon numbers of the alkyl radicals changes from nearly zero to -0.36.