Electrochemical impedance spectroscopy (EIS) at fixed potential was performed with gold electrodes covered with self-assembled monolayers (SAMs) of derivatized thiols on gold including mercaptopropionic acid (MPAc), mercaptododecylamine (MDAm), mercaptohexadecanoic acid (MHAc) and hexadecanethiol (HDT) using electrochemically inactive electrolytes. A simple equivalent circuit of the total interfacial capacitance in series with the solution resistance was found to fit the impedance spectra. Dissociation of acidic groups and protonation of basic groups on the surface give rise to an increase of the total interfacial capacitance which can be monitored by means of ac impedance. The interfacial capacitance changes with pH reflect a titration curve and this allows for an estimation of surface pK-values. The magnitude of the changes of the capacitances due to ionization were dependent on the hydrocarbon chain length of the monolayer and on the ionic strength. In agreement with other techniques, surface pK values were found to be significantly different as compared with the corresponding molecular moities dissolved in aqueous solution.