Electrochemical impedance spectroscopy was successfully employed to investigate ionic permeability into self-assembled monolayers (SAMs) deposited on a Au(111) single crystal in propylene carbonate electrolyte solutions. The ionic permeability strongly depends on the applied de potential, implying the existence of different potential dependent phases in monolayers exposed to propylene carbonate solutions. Increasing the temperature changes the ionic permeability of the SAM irreversibly. This is suggested to be due to changes in film structure due to Ostwald ripening. An appropriate equivalent circuit for characterization of the ionic conductivity of these self-assembled monolayers was developed.