Density functional theory (DFT) and G3-type (G3(MP2)-RAD) composite calculations were performed on a series of substituted 3-phenylpropyl radicals, to determine the relative importance of fragmentation and cyclization reactions in the chemistry of such species. Our studies indicate that cyclization is generally the more important of these reactions, with exceptions where fragmentation yields highly stabilized benzylic species. The energetic barriers for the cyclization reactions (enthalpies of activation) were found to be determined largely by the stability of the reactant radical and to a lesser but significant extent, by steric factors. Polarity effects in the transition state (modeled by SOMO-LUMO gaps of the products) appear to be less important. The data obtained indicated that the addition of benzyl radical to alkenes may be considered to be irreversible, but calculations for α-substituted styrenic systems indicate that reversibility of addition may become a factor in dilute polymerizing solutions for select systems.