The acid-base equilibrium of omega-functionalized alkanethiol monolayers on Au(lll) has been studied using the change in double-layer capacitance accompanied by the protonation-deprotonation of omega-terminals. The pK of omega-carboxyl alkanethiols on Au(lll) increases by four pH units. The shift becomes greater as the alkyl chain length increases. The same magnitude of the pK shift to the acidic side occurs in the monolayers of aminoethane thiol. The surface pK varies little with the electrode potential, whereas the increase in the supporting electrolyte concentration slightly diminishes the pK shift. The drawn-out shape of the titration curves is consistent with the mean-field model taking account of the repulsive interaction between adsorbed molecules, indicating the significance of the strong electrostatic repulsion between adsorbed thiol molecules in its charged states. The magnitude of the pK shift is, more than that predicted by the mean-field model, however, and suggests the considerable contribution from other factors that stabilize the uncharged state, for example, hydrogen bonding and low dielectric constant in the vicinity of the self-assembled monolayer. The possibility of counterion binding when the degree of deprotonation is large has been suggested.