The impedance of a Au(111) electrode modified by three different self-assembled monolayers has been studied over a wide frequency range in aqueous NaClO4 solutions. The impedance characteristics of the bare electrode are compared with those of the same electrode modified with decanethiol, ω-hydroxydecanethiol, and 4‘-hydroxy-4-mercaptobiphenyl. In the case of the decanethiol system the interfacial impedance can be represented as a capacitor due to the self-assembled monolayer in series with the solution resistance. However, for the latter two systems, the impedance behavior is more complex with a high resistance in the monolayer parallel to the expected capacitance. This behavior is quite different than that discussed to date in the literature and assumed in the interpretation of voltammetric data for modified electrodes. The parameters for these systems are reported here as a function of electrolyte concentration at the point of zero charge on the bare Au(111). The consequences of the present observations are discussed with respect to the phenomena observed for these systems.