The concept of an apparent effective thickness of a self-assembled monolayer is established and assessed with the measurements of ac impedance spectroscopy in different concentrations of Fe(CN)(6)(3-)/Fe(CN)(6)(4-) by alkanethiol self-assembled monolayers on a gold electrode. The complex-plane impedance plots for SAMs in solution of various concentrations of Fe(CN)(6)(3-)/Fe(CN)(6)(4-) exhibit are shapes. The apparent resistance (R-2), obtained from the simulation, decreased with increasing concentration of the redox couples. It is determined that the apparent effective thickness of the SAMs is lower than the length of chain of the alkanethiol. Defect sites may exist at the alkanethiol monolayers.