Potential scanning ellipsometry has been used to examine the wavelength and potential dependence of the optical constants of the lead layer formed on Au by underpotential deposition from Pb2+ solution. The two main current peaks in the voltammetry curves of the system can be identified with two different states of the surface film. Neither state has optical constants resembling those of bulk Pb. Both films have an ellipsometrically determined thickness of I.5.Å. The large wavelength dependence of the optical constants of the films indicates a strong interaction of the lead adlayer with the surface layer of the Au substrate. The ellipsometric data are consistent with a model in which the layer formed on the surface at less cathodic potentials is primarily ionic and at cathodic potentials undergoes a transition to a metal-like lead adlayer. © 1975, The Electrochemical Society, Inc. All rights reserved.