The optical properties of a series of nonmetallic and metallic surface films of ca monolayer thickness, which are adsorbed on metal electrodes, have been investigated by means of in situ differential specular reflection spectroscopy over the photon energy range 1.5–5.5 eV. Adsorbed O and oxide layers on Pt, Au and Ni substrates exhibit intense charge-transfer absorption bands (α 105–106 cm–1). The adsorbed H-atom layer on Pt produces an “inverse electroreflectance effect”. The effects of adsorption of monolayer Ag atom films on the oxidation of Pt electrodes were studied optically. The optical properties of very thin Ag films on Pt differ markedly from those of bulk silver and very rapidly with increasing film thickness.