The adsorption of 1,2-benzenedithiol (1,2-BDT) in benzene on gold and silver surfaces has been investigated by infrared reflection-absorption (IRA) spectroscopy, quartz crystal microbalance (QCM), and ellipsometry. The IRA spectral data were found to be consistent with those of QCM and ellipsometry. On both metals, 1,2-BDT was chemisorbed by forming two metal-sulfur bonds after deprotonation. Although monolayered film was formed exclusively on gold, multilayered film could also be formed on silver. The orientation on gold changed little with respect to the surface coverage, but structural change from vertical to tilted orientation occurred on silver at the fractional surface coverage near 0.6. The tilt angles of benzene ring were estimated to be ca.51 degrees on gold and 38 degrees on silver with respect to the surface normal. From the analysis of the IRA peak intensities, the free energy of adsorption, Delta G(a)(0), under submonolayer coverage limit was estimated to be -27.3 kJ/mol on gold and -39.6 kJ/mol on silver, suggesting that adsorption on silver would be energetically more favorable than that on gold. The multilayered film formed on silver consisted of aggregates of randomly oriented silver thiolate.