The effect of the double layer on the ionization of acidic head groups in a self-assembled monolayer on a gold electrode is considered. A model is developed which assumes that the head group is located in a region of low dielectric constant adjacent to the oillike monolayer. Expressions are developed for the potential distribution in the interface and its differential capacity. It is emphasized that discreteness-of-charge plays an important role in determining the local potential at the ionizable head group. The results obtained are discussed with respect to other models and experimental data presented recently in the literature.