The possibility that carotenoid radicals react with oxygen to form chain-carrying peroxyl radicals has been postulated to account for the reduction in antioxidant effetiveness displayed by some carotenoids at high oxygen concentrations. The primary objective of the work described in this paper was to measure the rate constants for oxygen addition to a series of carotenoid radicals and to examine any influence of carotenoid structural features on these rate constants. Laser flash photolysis has been used to generate long-lived carotenoid radicals (PhS-CAR radical) derived from radical addition reactions with phenylthiyl radicals (PhS radical) in benzene. The PhS-CAR radical radicals are scavenged by oxygen at rates that display a moderate dependence on the number of conjugated double bonds (ndb) in the carotenoid. The rate constants range from approximately 10(3) to approximately 10(4) M- 1 s- 1 for ndb = 7-11. The data also suggest that the presence of terminal cyclic groups may cause an increase in the rate constant for oxygen addition.