The monolayer of bis(arylgalvinol)-substituted alkyl disulfide and mixed monolayers with decanethiol (DT) on a polycrystalline gold electrode were characterized in an aqueous solution by means of cyclic voltammetry and in situ electroreflectance (ER) spectroscopy. Both neat and mixed monolayers exhibited a quasi-reversible redox response of galvinol/galvinoxyl radical couple (GalH/Gal(.)) at the same formal potentials. The ER signal involved responses due to both the redox reaction and the Stark effect on Gal(.). The ER spectrum of the redox reaction showed a difference absorption spectral feature, and it was found that the absorption bands of both immobilized GalH and Gal(.) are red shifted in comparison to their absorption bands measured in organic solvents. The ER spectral profile for the redox reaction was unaffected by mixing with DT, while the contribution of Stark effect on Gal(.) to the ER signal was significantly suppressed by increasing the amount of DT in the monolayer. The amount of immobilized GalH increased to larger than that of the neat monolayer as the mixing ratio of DT was increased up to 0.65, though further increases in the ratio resulted in a decrease in the amount of immobilized GalH. The increased mixing of DT narrowed the width of the CV peaks, consistent with an accelerated electron-transfer rate constant for the mixed monolayers. These results suggest that the neat monolayer may be disordered, while DT may mix well with the disulfide on the Au surface and force the monolayer to be more well-ordered.