The ionic insulating properties of n-alkylthiol self-assembled monolayers (SAMs) and a related fluorinated SAM are characterized by electrochemical ac impedance spectroscopy. CH3(CH2)(n)S/Au (n = 7-15) SAMs prepared via prolonged incubation in a thiol solution obey the Helmholtz ideal capacitor model and thus are virtually defect-free. The electrochemical properties of SAM-coated electrodes are compared with those of a bare gold electrode. The phase angle at an ion-diffusion-related frequency (1 Hz) is greater than or equal to 88 degrees and is independent of the electrolyte concentration. The thiol chains act gsa dielectric material whose impedance (modulus) is 10(5) Omega cm(2) at 1 Hz. Both the phase angle and the complex dielectric constant in the low-frequency region are good indicators of the film's ability to act as an ionic insulator.