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)(*) has a smaller O(2) affinity than the linoleic acid radical (LA(*)). Formation of the addition adduct radical ROO-CAR(*) is energetically favorable, and it has an even smaller tendency to react with O(2) than CAR(-H)(*). Comparatively, ROO-beta-CAR(*) is less likely to react with O(2) than ROO-LYC(*). Both the hydrogen abstraction and addition radicals (CAR(-H)(*) and ROO-CAR(*)) react readily with a second ROO(*) radical via either hydrogen abstraction or addition.