The adsorption of thiols on different metal surfaces has been studied using potentiometric measurements. The adsorption is a complex process that comprises a few steps. The first involves negative charge transfer to the electrode surface as evidenced by a sharp negative shift of the open circuit potential (ocp). The charge that is accumulated on the electrode surface is discharged through a reduction process that involves either water or oxygen reduction. The rate of the discharging process depends strongly on the overpotential of the electrode material for water reduction, the medium, and to some extent, on additional functional groups that the thiol bears. A detailed general mechanism that is in agreement with our findings is proposed and discussed. (C) 2002 Elsevier Science B.V. All rights reserved.