Simple π Hückel theory has been applied to radicals produced by unimolecular decomposition and by metathetical reactions of methyl radicals, and a correlation has been found between the A-factors of these reactions and the delocalisation and π-electron energies, respectively, of the radicals; the result of hybridisation changes causes a stiffening of the transition state. Activation energies of some of the abstraction reactions can be correlated with the electron density on the hydrogen atom being removed, calculated by σ-framework Hückel theory. The structure of small radicals is discussed, and the implications of the calculations are applied to inhibited pyrolysis and oxidation.