Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were used to examine four dithiol molecules, including 1,6-hexanedithiol, 1,9- nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol, adsorbed on well-ordered Pt(111) electrodes in 0.1 M HClO4. The open-circuit potential (OCP) of Pt(111) electrodes decreased substantially from 0.95 to 0.3 V (versus reversible hydrogen electrode) upon the adsorption of dithol molecules, which indicates that these adsorbates injected electrons into the Pt electrode. For all dithiol molecules, ordered adlattices of p(2 x 2) and (root 3 x root 3) R30 degrees were formed when the dosing concentration was lower than 150 mu M and the potential of Pt(111) was more negative than 0.5 V. Raising the potential of Pt( 111) from 0.1 to 0.4 V or more positive values could transform p( 2 x 2) to (root 3 x root 3) R30 degrees before it turned disarray. The insensitivity of the structure of dithiol adlayers with their chemical structures was explained by upright molecular orientation with the formation of one Pt- S bond per dithiol molecule. This molecular orientation was independent of the coverage of dithiol molecules, as nucleation seeds produced at the beginning of adsorption were also constructed with p(2 x 2). The triangular- shaped STM molecular resolution suggested 3-fold binding of sulfur headgroup on Pt(111). All dithiols were adsorbed so strongly on Pt( 111) electrodes that switching the potential negatively to the onset of hydrogen evolution in 0.1 M HClO4 or water reduction in 1 M KOH could not displace dithiol admolecules.