The adsorption of 5-mercapto-1-phenyl-tetrazole (5-PhTT) inhibitor on Cu(111) electrode surface was studied in sulfuric acid solution by using surface sensitive techniques (potentiodynamic scanning tunneling microscopy (EC-STM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES)). The composition of the protective layer was subsequently analyzed by Auger electron spectroscopy and X-ray photoelectron spectroscopy. Using the so-called potentiodynamic scanning tunneling microscopy mode, it was possible to follow the inhibitor film formation. In the double-layer potential region, the high-resolution STM imaging reveals details of the adsorbed layer and the molecular structure of individual molecules. Our in situ STM results were combined with quantum chemical calculations, which gave information about 5-MPhTT molecule size and symmetry. The results were analyzed and discussed. (C) 2003 Elsevier Ltd. All rights reserved.