The adsorption properties and chemical reactivity of semiconductor surfaces are of great interest from both a fundamental and an applied point of view. We are carrying out since many years a systematic investigation on the adsorption of various organic and organometallic species on semiconductors, in particular the silicon (111) cleaved surface. The UHV technique we primarily use is synchrotron radiation photoemission. The main results of this research line are: the complete characterization of the adsorption properties of various classes of organic systems (alkanes, alkenes, alkynes, aromatics, alcohols); the discovery of a surprisingly high reactivity of the Si surface with respect to aromatic molecules; the characterization for the first time of the adsorption properties of organometallic systems on Si (carbonyls and cyclopentadienides of various transition metals); the possibility of using unmonochromatized synchrotron radiation (“white light”) to induce chemical reactions on the Si surface saturated with molecular adsorbates, in particular the capability of depositing metallic films using organometallic species as precursors.