The adsorption of three different thiol-based additives, commonly used in sulfuric acid copper plating baths, on Cu(111) has been studied by cyclic voltammetry and in situ scanning tunneling microscopy. Adsorption rates were determined and the adlayer growth was monitored for various thiol concentrations. The observed growth peculiarities were tentatively explained by the lifting of a sulfate-induced reconstruction of Cu(111) and, in the case of 3-mercaptopropionic acid, by a bilayer formation. (c) 2007 The Electrochemical Society.