The selectivity of the reaction of benzoyloxy radicals with styrene has been examined. Analysis of the products formed when benzoyl peroxide is decomposed in styrene at 60°C in the presence of a radical trapping agent (2,2,6,6-tetramethylpiperidine-1-oxyl) shows that the reaction of benzoyloxy radicals with styrene proceeds with 80% “tail” addition, 5% “head” addition, and 15% aromatic substitution. Phenyl radicals (formed by decarboxylation of benzoyloxy radicals) also may add to either the double bond or the aromatic ring of styrene. The importance of the above-mentioned processes to the structure and properties of benzoyl-peroxide-initiated polystyrene is discussed.