Self-assembled monolayers (SAMs) of o-xylene-alpha,alpha＇-dithiol (o-XDT) on a Au(111) surface have been characterized by means of reflection-absorption infrared (RAIR) spectroscopy, ellipsometry, quartz crystal microbalance (QCM), and atomic force microscopy (AFM). The RAIR spectral data revealed that o-XDT should adsorb on gold as a deprotonated form with its benzene ring to assume a perpendicular stance with respect to the gold substrate. The ellipsometric thickness of the o-XDT SAMs was 8.3 Angstrom, comparable to the prediction from the RAIR spectra. The QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at -28.9 kJ/mol. The limiting surface coverage was determined from the QCM data to be 5.0 x 10(-10) mol/cm(2), corresponding to an area per adsorbate of 33 Angstrom(2). The AFM image showed a highly ordered and close-packed (2 x 2) structure with an area per adsorbate of 29 Angstrom(2), which was quite close to the value obtained from the QCM data.