AbstractThe dynamics of action of ubiquinol as an antioxidant against lipid peroxidation was reinvestigated and compared with that ofα‐tocopherol. It was found that ubiquinol was 2.5 and 1.9 times more reactive thanα‐tocopherol toward phenoxyl and peroxyl radicals, respectively, at 25°C in ethanol and that it was capable of donating two hydrogen atoms toward oxygen radicals but that the apparent stoichiometric number decreased in the inhibition of lipid peroxidation, to even smaller than 1, due to its autoxidation. The autoxidation of ubiquinol proceeded even in the micelles and liposomal membranes in aqueous dispersions as well as in organic homogeneous solution. The apparent antioxidant activity of ubiquinol was smaller than that ofα‐tocopherol against lipid peroxidation in organic solution as judged from either rate of oxidation or duration of inhibition period. They exerted similar antioxidant potency against lipid peroxidation in the membranes and micelles in aqueous dispersions. The combination of ubiquinol andα‐tocopherol was suggested to be effective.