We investigated the adsorption properties of methanol and ethanol on Si(111)2 x 1 as a function of temperature in the range 80-300 K by means of synchrotron radiation photoemission of core and valence levels. A physisorbed state is characterized at 80 K, whereas at 300 K an alcohoxy species is formed. White-light exposures promote a decomposition reaction which leads to carbon on the Si surface, either on elemental form or as CH(x) groups.