Chain-breaking reactions against lipid peroxidation performed by carotenes, including beta-carotene (beta-CAR) and lycopene (LYC), have been studied using density functional theory. We chose linoleic acid (LAH) as the lipid model and examined two mechanisms: hydrogen abstraction and addition. Our computed reaction diagrams reveal that the addition mechanism is able to offer a larger extent of chain-breaking protection than hydrogen abstraction. In the case of hydrogen abstraction, the resulting carotene radical CAR(-H)(center dot) has a smaller O-2 affinity than the linoleic acid radical (LA(center dot)). Formation of the addition adduct radical ROO-CAR(center dot) is energetically favorable, and it has an even smaller tendency to react with O-2 than CAR(-H)(center dot). Comparatively, ROO-beta-CAR(center dot) is less likely to react with O-2 than ROO-LYC center dot, Both the hydrogen abstraction and addition radicals (CAR(-H)(center dot) and ROO-CAR(center dot)) react readily with a second ROO center dot radical via either hydrogen abstraction or addition.