The ionic permeability of alkanethiol self-assembled monolayers (SAMs) chemisorbed on gold is studied using ac impedance spectroscopy in the absence of redox active species. CH3(CH2)(n)S/Au (n = 7-15) SAMs behave as ionic insulators until a critical potential, V-c, is reached or exceeded. At potentials more cathodic than V-c, SAMs are no longer ionic insulators and a significant change in the phase angle is associated with ion penetration in the low-frequency region. Ve is chain length dependent and is observed at potentials (-0.15 to -0.35 V vs Ag/AgCl) that are considerably more anodic than the alkanethiol electrodesorption potential. The relaxation frequency of trans-SAM ion migration (4-100 Hz) can be calculated from fitting of the impedance data to an appropriate equivalent circuit or from Bode phase plots.