The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule-electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of n-alkanethiolates (S(CH2)(n-1)CH3 SCn with n = 8, 10, 12, or 14) of the form Ag-Ts-SCn//GaOx/EGaIn (a protective thin (similar to 0.7 nm) layer of GaOx, forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (R-s, which includes the SAM-electrode contact resistance), the capacitance of the SAM (C-SAM), and the resistance of the SAM (R-SAM). A plot of R-SAM vs n(C) yielded a tunneling decay constant beta of 1.03 +/- 0.04 n(C)(-1), which is similar to values determined by DC methods. The value of C-SAM is similar to previously reported values, and Rs (2.9-3.6 x 10(-2) Omega.cm(2)) is dominated by the SAM top contact resistance (and not by the conductive layer of GaOx) and independent of n(C). Using the values of R-SAM, we estimated the resistance per molecule r as a function of n(C), which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions.