The adsorption behaviour of 2-thiothymine and 4-thiothymine on a Au(1 1 1) single crystal electrode has been studied using cyclic voltammetry and X-ray photo electron spectroscopy For both thio derivatives the adsorption region is restricted due to the onset of reversible oxidization to 2 2'-bis(1H-5-methylpyrimidin-4-one-2-yl)-disulphide or 4 4'-bis(1H-5-methylpyrimidin-2-one-4-yl)-disulphide at anodic potentials Two different orientations of adsorbed 2-thiothymine have been observed Between -350 my and -700 mV versus Ag/Ag+ the molecule is solely chemisorbed via its sulphur atom and adopts an upright orientation towards the surface However at more negative potentials 2-thiothymine is reoriented into a slightly tilted position interacting via its S N and O atoms with the surface In contrast 4-thiothymine exhibits only one adsorption geometry Between -300 mV and -700 mV versus Ag/Ag+ it is chemisorbed via sulphur and nitrogen adopting a slightly tilted position At -950 mV versus Ag/Ag+ 4-thiothymine is irreversibly reduced The sulphur substituent is eliminated and covers the substrate (C) 2010 Elsevier Ltd All rights reserved