The electrostatic description of an electrode modified by a self-assembled monolayer containing redox centers is considered. The model assumes that there is a Stern layer containing hydrophobically bonded water molecules between the monolayer and the electrolyte solution, and also considers the stabilizing effects of the adjacent metal and solution in estimating the local potential at the chargeable groups in the monolayer. The local potential and interfacial capacity are estimated as a function of electrode potential, and the results are compared with those from a simpler model.