A complete inspection of the capabilities of reflectance anisotropy spectroscopy (RAS) in studying the adsorption of molecules or atoms on the Si(001)-(2 x 1) surface is presented. First, a direct comparison between RA spectra recorded on the clean Si(001)-(2 x 1) and the corresponding topography of the surface obtained using scanning tunneling microscopy (STM) allows us to quantify the mixing of the two domains that are present on the surface. Characteristic RA spectra recorded for oxygen, hydrogen, water, ethylene, benzene are compared to try to elucidate the origin of the optical structures. Quantitative and qualitative information can be obtained with RAS on the kinetics of adsorption, by monitoring the RA signal at a given energy versus the dose of adsorbate; two examples are presented: H-2/Si(001) and C6H6/Si(001). Very different behaviours in the adsorption processes are observed, making of this technique a versatile tool for further investigations of kinetics. (c) 2006 Elsevier B.V. All rights reserved.