A model for the potential distribution at the interface between a gold electrode modified by a self-assembled monolayer, in which some units terminate in an ionizable head group, and an electrolyte solution is presented. Estimation of the local potential at an ionizable head group is based on the hexagonal array model by Barlow and Macdonald in which the role of the nearby dielectric discontinuities is considered. The effect of the electrode potential on ionization of the head groups and on the differential capacity of the interface is estimated by using this model, and the results are compared with those ignoring the three-dimensional nature of the potential distribution and those obtained from the cutoff disc model. Capacity data obtained recently at gold electrodes modified by mercaptopyridine and mercaptoaniline are analyzed on the basis of the model presented.