The formation and organization of self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) has been studied on mercury surfaces. The organization process has been examined by cyclic voltammetry and chronoamperometry. The study confirms the occurrence of a two-dimensional (2D) condensation at more positive potentials than those of the cathodic/anodic adsorption peaks at about -0.2 V. The condensed phase is destroyed at more negative potentials. Charging current measurements as well as electron transfer experiments indicate the formation of a thin film monolayer. This film has no effect on the electron transfer rate constant of an outer-sphere redox couple but it shows a strong ability to inhibit electroreduction of dioxygen as it is characteristic of condensed organic monolayers. Cu2+ is able to permeate the 6MP film, giving a current only three-fold lower than that at a bare hanging mercury drop electrode (HMDE) and with no-linear diffusion characteristics.