The effects of carotenoids on the oxidation of refined edible oils, especially the prooxidant effect, were examined. To study the roles of carotenoids in the oxidation of oils, , β-carotene (β-car) and its decomposition or oxidation products were added to test oils [methyl linoleate, soybean triglyceride or refined rapeseed oil] and allowed to oxidize with exposure to light or darkness. In the autoxidation of test oils with β-car the prooxidant effect of, β-car, was frequently observed, especially when the test oils were highly oxidized before the addition of β-car. The addition of, β-car usually improved the stability of test oils against photooxidation. The prooxidant effect of β-car was considered to arise not from β-car itself or the highly oxidized decomposition products of β-car, but rather from intermediate compounds during its oxidation. These compounds function as initiators for chain reactions of lipid oxidation. However, carotenoids may possibly not exert marked prooxidant effect on ordinary edible plant oils since considerably greater amounts of tocopherols than carotenoids are pre-sent and tocopherols have strong antioxidant effect on edible oils and residual carotenoids.