In our study, we report on the action of adsorbed thymine molecules during the copper deposition on Au(111) electrodes. We could demonstrate that thymine molecules act as a catalyst for the Cu-UPD however as an inhibitor for the bulk deposition. The higher the thymine surface concentration is, the faster is the monolayer deposition (UPD) of copper, which could be shown by chronocoulometric studies. During the deposition of copper the thymine molecules reorientate and change their adsorption state from a physisorbed state on the blank Au(111) substrate to a chemisorbed one on top of the copper monolayer. During the reorientation an intermediate step could be recognized which is probably the adsorption of a thymine-copper complex. Both the formation and the dissolution process of copper obey a nucleation and growth mechanism. The catalytical action of thymine molecules can be explained for energetical reasons. Due to the chemisorption of thymine molecules on top of the copper layer the potentials of zero charge of Au(111) and copper become close together, thereby reducing the surface strain between the two metals. (c) 2006 Elsevier B.V. All rights reserved.