The adsorption and phase formation of 6-thioguanine (6TG) on the mercury/aqueous electrolyte interface has been studied using cyclic and ac sensitive voltammetry and capacitance-potential curves. The molecules form two highly ordered monolayers depending on the potential. At high potential values, a region can be distinguished where the molecules are chemisorbed and are able to self-assemble and form a self-assembled monolayer (SAM). They remain in that state up to potential values where the reductive desorption of the whole monolayer takes place. The voltammetric peaks corresponding to this reduction process together with those for SAM formation are characteristics of a 2D-condensed phase transition process. These peaks separate the region where the SAM is stable from the potential region where a physisorbed state exists. This latter state is temperature dependent and shows the characteristic features of an ordered state at low temperatures. A study of the desorptive reduction process as a function of the solution pH indicates that in the pH region above the pK value for the -SH group of 6TG, the SAM can no longer be formed. A comparison with the behaviour of its analogue 6-mercaptopurine (6MP) allows us to find some differences in the chemistry of these molecules in relation with the SAM formation capability and in the interactions responsible for their stability. (C) 2003 Elsevier B.V. All rights reserved.