The electrode reaction of methylene blue (MB) at a polycrystalline gold electrode modified with a self-assembled monolayer of n-dodecanethiol was analyzed by using voltammetric and electroreflectance (ER) techniques. The results for the gold electrode modified with a dodecanethiol monolayer were compared with the data obtained at a bare gold electrode and a gold electrode modified with butanethiol, thiophene, or sulfur. The ER spectrum of MB at the gold electrode modified with dodecanethiol monolayer was in accord with the difference absorption spectrum between reduced and oxidized forms of MB in the solution. The ER spectrum of MB at the bare gold electrode had a different structure. We concluded that MB molecules were partitioned into the dodecanethiol monolayer interior on the gold electrode surface. The MB molecules, however, were not in direct contact with the gold surface when the electron transfer takes place.