Solvent effects on the activities of aryloxyl‐radical (aroxyl‐radical) scavenging and singlet‐oxygen (1O2) quenching reactions by vitamin E (α‐tocopherol) were studied with stopped‐flow and laser spectroscopies together with density functional calculations. Partial electron transfer from α‐tocopherol plays an important role in an early step of each of these antioxidant reactions. To shed light on the solvent effects, we used ethanol as the reference solvent and added water to it. The rate constant of the aryloxyl‐radical scavenging reaction by α‐tocopherol increased monotonously by adding water to the ethanol solution, because the oxidation potential of α‐tocopherol decreases as water is added. Although the rate constant of the 1O2 quenching reaction similarly increased with increasing water concentration from 0% to 10% v/v, it reached a maximum at about 10% v/v and decreased in the range 10%–20% v/v. The suppression of the 1O2 quenching in the 10%–20% v/v range could be attributed to complexes forming between 1O2 and the water molecules. On the basis of the present results, we inferred how the antioxidant activities of α‐tocopherol depend on its position in biomembranes.

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