We report on an experimental investigation of benzene adsorption on single-domain Si(001)-(2×1) by means of surface sensitive optical spectroscopies which are reflectance anisotropy spectroscopy and surface differential reflectivity spectroscopy combined with scanning tunneling microscopy. These optical techniques are sensitive to the number of occupied dimers and permit us to demonstrate that benzene molecules are bound to slightly less than one dimer out of two. We investigate the modification of optical spectra on the benzene-saturated surface versus time and conclude that no thermally induced geometry conversion of benzene molecules is observed.