A procedure to evaluate the surface dipole potential X of thiol and disulfide self-assembled monolayers (SAMs) is described. The procedure consists of self-assembling the monolayers on a hanging mercury drop electrode and of measuring the charge involved in a progressive expansion of the mercury drop. This measurement is then combined with an estimate of the charge density q experienced by diffuse layer ions, obtained by measuring the diffuse layer capacitance of the SAM at different electrolyte concentrations by electrochemical impedance spectroscopy. These chi measurements, combined with chronocoulometric measurements of the total charge density sigma(M) against potential, indicate that SAMs of tetraoxyethylene glycol-D,L-alpha-lipoic acid ester (TEGL), 2,3-di-O-phytanyl-sn-glycerol-1-tetraoxyethylene glycol-D,L-alpha-lipoic ester (DPTL), and trioxyethyleneoxythiol (EO3) on mercury may undergo a reversal in the surface dipole potential of their polyoxyethylene chain with a change in the interfacial electric field. Moreover, TEGL and EO3 form stable SAMs without electron transfer to the metal, while no such conclusion can be drawn for DPTL.