A spectro-voltammetric analysis, in which an in situ time-resolved absorption spectroscopy with a 2 ms time resolution was combined with a fast-scan CV or with a potential step method, is introduced as a useful method to illuminate the unique redox properties of layered J-aggregate of cyanine dyes, as organized on a cysteamine-modified Au (111) surface. The in-situ time-resolved absorption spectra of the J-aggregate provided direct spectroscopic evidence for the reversible J-aggregate oxidation and a convenient way to monitor the oxidized mole fraction in the J-aggregate in real time. The potential step method enabled us to observe the reversible redox equilibrium of the layered J-aggregate. The experimental data were analyzed in the framework of the ideal Nernst equation to extract the standard potential for the J-aggregate oxidation and to examine the extent of non-ideality caused at higher oxidation levels due to the repulsive interactions between dye positive holes. The standard potential for the J-aggregate oxidation in the model system was substantially more negative than that known for the monomer by-0.14V. Implications of the results for spectral sensitization phenomena are also discussed.