dIon penetration into a series of omega-functionalized X(CH2)(n)S/Au (X = CH3, OH, or CO2H, and n = 15) self-assembled monolayers (SAMs) and a partially fluorinated (CF3(CF2)(7)(CH2)(2)S/Au) SAM has been investigated by electrochemical ac impedance spectroscopy in the absence of a redox probe. SAM permeability is revealed by the behavior of the phase angle at frequencies of less than similar to 50 Hz, the frequency domain characteristic of diffusion processes. The permeability of these omega-functionalized SAMs, as a function of an applied potential, falls into two regimes. One regime corresponds to a state where the SAM is an ionic insulator and is well described by the Helmholtz capacitor model. The second regime begins when a critical applied potential, V-c, is exceeded. V-c corresponds to the applied potential at which ion penetration into the SAM is activated. For a chain length of 15 carbon atoms, the chemical nature of the terminal group X greatly influences the value of V-c, where V-c is +0.25 V (vs Ag/AgCl) for X = OH, +0.15 V for X = CO2H, -0.35 V for X = CH3, and -0.25 V for the fluorinated SAM. A hydrophilic SAM/electrolyte interface, rather than a hydrophobic one, is more readily transformed into a form which favors ion/water penetration into the SAM. The potential-induced transformation described here is of importance to the application of SAMs in biosensors and molecular electronic devices.