Polarization modulation Fourier transform infrared reflection spectroscopy is used in situ to obtain vibrational spectra from a copper electrode immersed in a cyanide solution. For 0.1M solutions, Cu(CNn+in-, where n = 1–3, solution species and CuCN surface species are observed. In solutions much less concentrated than 0.01M, the vibrational frequencies of the adsorbed species are potential dependent. The relative amounts of species are controlled by the potential dependent Cu+ ion concentration and the appropriate complex equilibria. © 1987, The Electrochemical Society, Inc. All rights reserved.