Mathematical treatments of continuous-flow column electrolytic spectroscopy for the two-step one-electron transfer reaction are presented. The relationship between the absorbance and the apparent number of electrons in electrolysis, which is independent of electrolytic efficiency, is useful for the analysis of quasi-reversible as well as reversible redox reactions. The theory has been applied to the analysis of the redox reaction of 9,10-anthraquinone-2,7-disulfonate, which exhibits totally reversible characteristics in continuous-flow column electrolysis and also in cyclic voltammetry at a column electrode in stationary solution. The redox potentials of the two successive electron transfers and the absorption spectra of the semiquinone intermediate in aqueous solution have been successfully determined. The redox reaction of 2,6-dimethyl-1,4-benzoquinone, which is irreversible (or quasi-irreversible) in cyclic voltammetry at a conventional solid electrode, has also been characterized from the thermodynamic aspects.