The mechanism of the autoxidation of reduced flavin and quinone compounds was investigated through semiquantitative analysis of voltammetric waves of the reduction of dioxygen (O-2) catalyzed by flavin and quinone adsorbed on electrode surfaces, where the redox equilibrium among the oxidized, (flavo)semiquinone and fully reduced states is easily controlled by the electrochemical method. The analysis provided evidence that the semiquinone radical plays a predominant role in the autoxidation. The generation of a superoxide anion radical as a product of the one-electron transfer from the semiquinone to O-2 was confirmed by the reduction of ferricytochrome c. The fact that the catalytic reduction wave of O-2 increases with pH was ascribed to the increase in the semiquinone formation constant. The mechanism of the reoxidation of reduced flavoprotein glucose oxidase with O-2 was also examined. The result supports the semiquinone-dependent one-electron transfer as the mechanism.