The ac impedance behavior of siloxane-anchored self-assembled monolayers (SAMs) bonded to a silicon substrate has been examined. These results are correlated with ellipsometry and wettability data. The films that were studied include homogeneous alkylsiloxane SAMs ((SiO)3Si(CH2)nCH3, n = 7, 11, 17, 21), mixed chain length alkylsiloxane SAMs, the [16-(chloroacetoxy)hexadecyl]siloxane SAM ((SiO)3Si-(CH2)16O(CO)CH2Cl), and two derivatives thereof where the Cl is replaced by a thiol (decanethiol or p-nitrothiophenol). For nonpolar alkylsiloxane SAMs, the thicknesses measured by ac impedance and by ellipsometry are in close agreement and a reasonable interpretation is possible even for the mixed chain length systems. For thin films with polar functional groups, thickness estimates by ac impedance are problematic, since an accurate, independent measure of their relative permittivity is not readily available. The insulating properties of all these films can be evaluated from the frequency dependence of the imaginary component of the impedance responses. In highly insulating films, characteristic capacitance measurements are virtually frequency independent over the range of 5-10000 Hz, but with increasing imperfections, the characteristic capacitance values become more dependent on frequency. The quality of the films with regard to the extent of defects can thus be directly evaluated by ac impedance.