The adsorption of Benzenethiol and 1-Undecanethiol on polycrystalline copper and their ability to inhibit the Cu corrosion process for several days in strongly acidic solution were studied by XPS and EIS. Surprisingly, the SAM of aromatic thiols assured higher protection and higher stability than the SAM of long-chain alkylic thiols in spite of their lower thickness, thanks to a noticeable increase in the charge-transfer resistance observed by EIS after few hours of exposure to the electrolyte. XPS data revealed that the bond with the substrate is more stable in the case of Benzenethiol than with 1-Undecanethiol. This is probably due to the presence of strong ring interactions, which allows the SAM to assume a highly ordered structure. The higher stability of the aromatic SAMs appears to be more important than the layer thickness for substrate passivation. (C) 2011 Elsevier B.V. All rights reserved.