The effects of J-aggregation on the redox thermodynamics for a cationic cyanine spectral sensitizing dye are investigated. When adsorbed to the surfaces of cubic AgBr microcrystals, both the monomeric and J-aggregate forms of the cyanine dye can be reversibly oxidized with redox solutions containing ferricyanide or molybdicyanide complex. Diffuse reflectance spectra recorded for thin gelatin coatings of the dyed microcrystals after treatment with redox solution show distinct spectral bands associated with the dye and a stable, monooxidized dye radical ion, The fraction of dye oxidized can be accurately varied by simple adjustment of the electrochemical potential of the redox buffer solution. For the J-aggregated dye, the reflectance band for the dye shifts to shorter wavelengths and becomes significantly broadened with increasing fractional degree of oxidation. The formal oxidation potential for the adsorbed dye can be obtained from a Nernstian plot of redox-solution potential E vs log [oxidized dye]/[dye] as constructed from the reflectance spectral data. The results indicate that the one-electron oxidation potential of the monomer on cubic AgBr to be lower than that of the J-aggregate by 74 mV. The energy of the singlet excited state of the J-aggregate is calculated to be lower than that of the monomer by a much larger amount. For the cationic dye in this study, the aggregation-induced changes in redox potential result in dramatic differences in the comparative photoresponses of the monomer and aggregate dye systems.