beta-Carotene and other carotenoids are thought to exert disease preventive actions by scavenging reactive free radicals, but the mechanisms of these reactions are poorly understood. We detected products formed by reaction of beta-carotene with alkoxyl, and alkylperoxyl free radicals generated by thermolysis of azobis(2,4-dimethylvaleronitrile) (AMVN) in benzene. Analyses by atmospheric pressure chemical ionization mass spectrometry identified two previously unknown classes of beta-carotene oxidation products. Substitution products contain one AMVN-derived radical adduct group and result from hydrogen transfer from the carotenoid polyene followed by radical recombination. Addition products contain two AMVN-derived adduct groups and result instead from sequential radical additions to the polyene. These product structures provide the first mechanistic explanation for the radical scavenging reactions of carotenoids.