Detailed kinetic studies have been performed for the reaction of aroxyl (ArO center dot) radical with vitamin E (alpha-, beta-, gamma-, delta-tocopherol, TocH), ubiquinol-10, and related antioxidants in micellar solution, using a stopped-flow spectrophotometer. The second-order reaction rates (k(s)) obtained increased in the order of hydroquinone < tocol <delta-T6cH < ubiquinol-0 <gamma-TocH similar to beta-TocH < ubiquinol-10 <alpha-TocH at pH 4 similar to 9. The antioxidants which have lower oxidation potentials (E(p)) showed higher reactivities. The reaction rates obtained in micellar solution were pH dependent because of the dissociation of OH groups in the antioxidants. For instance, by comparing the k(s) values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate k(s1) (1.21 x 10(5) M(-1)s(-1)) for undissociated form, k(s2) (1.04 x 10(6) M(-1) s(-1)) for monoanion and k(s3) (0 M(-1)s(-1)) for dianion, and the pK(a1) and PK(a2) values (11.4 and 12.7) were determined. It was found that the relative ratio of K(s) values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biological activities for rat fetal resorption, rat haemolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed on the basis of the products of the k(s) values and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).