The barriers, enthalpies, and rate constants for the addition of methyl radical to the double bonds of a selection of alkene, carbonyl, and thiocarbonyl species (CH2=Z, CH3CH=Z, and (CH 3)2C=Z, where Z = CH2, O, or S) and for the reverse β-scission reactions have been investigated using high-level ab inito calculations. The results are rationalized with the aid of the curve-crossing model. The addition reactions proceed via early transition structures in all cases. The barriers for addition of methyl radical to C=C bonds are largely determined by the reaction exothermicities. Addition to the unsubstituted carbon center of C=C double bonds is favored over addition to the substituted carbon center, both kinetically (lower barriers) and thermodynamically (greater exothermicities). The barriers for addition to C=O bonds are influenced by both the reaction exothermicity and the singlet-triplet gap of the substrate. Addition to the carbon center is favored over addition to the oxygen, also both thermodynamically and kinetically. For the thiocarbonyl systems, addition to the carbon center is thermodynamically favored over addition to sulfur. However, in this case, the reaction is contrathermodynamic, addition to the sulfur center having a lower barrier due to spin density considerations. Entropic differences among corresponding addition and β-scission reactions are relatively minor, and the differences in reaction rates are thus dominated by differences in the respective reaction barriers.