It is well known that fractional monolayers of sodium can be electrolytically deposited on the Si-SiO2 interface, but the resulting electronic structure has not been fully determined. We present thermionic emission and photoemission measurements of uniformly coated interfaces which clearly show a reduction of the contact barrier due to an interface dipole layer. The photoelectric threshold, determined by the photoyield corrected for optical absorption Y/α, is reduced by sodium to a saturation value of 2.6 eV at a full monolayer coverage. This is consistent with the thermionic emission results which show an electronic barrier of 2.7 eV at the full monolayer coverage. A simple mathematical model is given to explain this dipole layer. © 1974 The Japan Society of Applied Physics.