The orders of relative tocopherol biopotency, scavenging activity of lipid peroxyl radicals, and quenching activity of singlet oxygen are as follows: α-tocopherol > β-tocopherol ≥ γ-tocopherol > δ-tocopherol. However, the reverse is the case for the activity to protect fats and oils from oxidation: α-tocopherol < β-tocopherol < γ-tocopherol < δ-tocopherol. The reason for this reverse has been studied by measuring weight gains of solutions including methyl linoleate and various antioxidants at daily intervals. Antioxidant activity is expressed as an induction period in days. Induction periods are compared with the second-order rate constants for scavenging of an aroxyl radical by the antioxidants. A plot of the logarithm of the secondorder rate constant vs. the induction period is found to give a good linear fit, and the slope is negative. From these results, it is considered that the larger the peroxyl-radical-scavenging activity of a tocopherol, the more favorable the production of the tocopheroxyl radical, and thus the more efficient the oxidation of fats and oils by the tocopheroxyl radical. This is not the case for tocopherols in vivo, because vitamin C and ubiquinol reduce tocopheroxyl radicals formed from tocopherols. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [605][606][607][608][609][610] 2005