The isotropic C-13 hyperfine splitting (hfs) constant, a(C-13(alpha)), of symmetrically alpha-substituted methyl radicals X(3)C(.) has been computed as a Boltzmann average over the out-of-plane bending vibration by ab initio calculations at the UMP2/DZP/TZP level of theory. Experimental values from ESR spectra are well reproduced by these calculations. The large variations in a(C-13(alpha)) observed experimentally upon alpha-substitution are due not only to structural changes but also to the electronic effect of the alpha-substituents. The latter effect is largely dominant with strongly electronegative substituents. The value of a(C-13) for X = F (269.8 G) is computed to be about twice that for X = OMe (150 G), whereas both radicals are computed to have a nearly tetrahedral structure. This confirms that in localized radicals care must be exercised in obtaining structural information comparing the experimental hfs constants of radicals bearing alpha-substituents with different electronegativity.