Accurate activation energies for 67 hydrocarbon radical addition and beta-scission reactions are calculated with the CBS-QB3 ab initio method. An extension of Benson's group additivity method to activation energies is introduced. The underlying hypotheses, that is, the group concept and the additivity approximation, are validated with ab initio data. Standard activation group additivity values are obtained from the ab initio calculations for reactions involving primary, secondary, tertiary alkylic, allylic, benzylic, and vinylic radicals. The proposed group contribution method yields accurate activation energies for radical addition and for beta-scission reactions. The effect of substituents on the carbon atoms of the reactive center on the activation energy can be as large as 95 kJ/mol for the adding radical, and 187 kJ/mol for the product radical of the P-scission. Non-nearest-neighbor effects such as gauche and cis interactions have an influence of less than 3 kJ/mol per interaction on the activation energies. However, for hydrocarbons that are heavily branched near the reactive center, these interactions can become important. (C) 2004 American Institute of Chemical Engineers.