Several examples are given on the investigation of adsorption of atoms and molecules on Si(001)2 × 1 in ultra-high vacuum, by means of two different surface optical spectroscopies, the Surface Differential Reflectance Spectroscopy and the Reflectance Anisotropy Spectroscopy. Hydrogen, water, oxygen, benzene and pyridine adsorptions are presented. Specific spectra are obtained, which are interpreted as a function of the mode of adsorption of the adsorbates. In particular, it is possible to discriminate between adsorption on the dangling bonds of the Si dimers and adsorption inducing a breaking of the dimers. Moreover, quantitative information is obtained from these optical techniques which yields the determination of the absolute number of surface Si atoms involved in the bonding.