The adsorption of 3-mercaptopropionic acid (MPA) on Au(111) in 0.1 M H2SO4 was studied by cyclic voltammetry, capacitance measurements and in situ scanning tunneling microscopy (STM). The formation of a self-assembled monolayer under electrochemically controlled conditions was monitored for the first time with an STM. A two-step adsorption process has been found, beginning with the adsorption of MPA to form a disordered film, which lifts the (22 x root3) reconstruction of Au(111), followed by an ordering of the MPA adlayer into two different dense structures. Concomitantly with the second step, formation of the well-known vacancy islands is observed. This formation is explained by an adsorbate-induced compression of the gold substrate. Two distinctly different structures of the dense MPA monolayer on Au(111) wc re observed. One (p x root3) structure with p = 3.6-4.8 and a second structure consisting of two unit cells of (2root3 x root7) and (2root3- x 4) symmetry. In part II, the effect of the MPA on the copper electrodeposition onto Au(111) is discussed. (C) 2003 Elsevier Ltd. All rights reserved.