Formation and reductive desorption of self-assembled monolayers of 4-mercaptophenol on mercury have been studied by using cathodic stripping voltammetry. Voltammetric peak shapes and integrated areas are consistent with a sequential reorientation of adsorbed molecules from a flat to a perpendicular orientation as their surface concentration increases. The rate of Ru(NH3)(6)(3+) reduction in the presence of full grown thiol monol ayers has been determined by impedance measurements. The frequency dispersion of the electrode admittance was found to obey Randles' behavior, though the parallel capacitance was observed to depend on the redox probe concentration when the monolayers became ionized at pH > 4. Non-ionized 4-mercaptophenol monolayers are more effective electron conductors than alkanethiol monolayers of the same thickness. After correcting for the diffuse layer potential drop, faster electron transfer rates are observed in the presence of ionized monolayers. This increase of reactivity correlates with results obtained from quantum chemical calculations performed on a model system, which predict an increase of donor-acceptor electronic coupling when the intervening molecular bridge becomes deprotonated. (C) 2005 Elsevier B.V. All rights reserved.