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In situ electrochemical scanning tunneling microscopy (STM) has been used to examine the structures of benzenethiol adlayers on Au(100) and Pt(100) electrodes in 0.1 M HClO4, revealing the formation of well-ordered adlattices of Au(100)-(root 2x root 5) between 0.2 and 0.9 V and Pt(100)-(root 2 x root 2)R45 degrees between 0 and 0.5 V (versus reversible hydrogen electrode), respectively. The coverage of Au(100)-(root 2 x root 5) is 0.33, which is identical to those observed for upright alkanethiol admolecules on Au(111). In comparison, the coverage of Pt(100)-(root 2 x root 2)R45 degrees - benzenethiol is 0.5, much higher than those of thiol molecules on gold surfaces. This result suggests that berizenethiol admolecules on Pt(100) could stand even more upright than those on Au(100). All benzenethiol admolecules were imaged by the STM as protrusions with equal corrugation heights, suggesting identical molecular registries on Au(100) and Pt(100) electrodes, respectively. Modulation of the potential of a benzenethiol-coated Au(100) electrode resulted in irreversible desorption of admolecules at E <= 0.1 V (vs. reversible hydrogen electrode) and oxidation of admolecules at E >= 0.9 V. In contrast, benzenethiol admolecule was not desorbed from Pt(100) at potentials its negative as the onset of hydrogen evolution. Raising the potential tendered deposition of more benzenethiol molecules before oxidation of admolecules commenced at E > 0.9 V. (c) 2006 Elsevier B.V. All rights reserved.