Deuterated methanol adsorption on Si(111)7 7 and Si(100)2 1 surfaces has been investigated, at room temperature, by synchrotron radiation photoemission and photostimulated desorption. Photoemission experiments as a function of methanol coverage show that deuterated methanol adsorption is dissociative on both surfaces and occurs via σ D) bond breakage and σ S i O bond formation. Site selectivity is displayed by the methoxy species on Si(111)7 7 surface, since the S 2 surface state related to the rest atoms is quickly quenched upon low coverage adsorption. The S 1 surface state is quenched only subsequently at much higher coverages. The photostimulated desorption pattern is similar for the two surfaces and strongly depends on the investigated energy range. D + is always the most abundant ion produced, but higher mass fragments are present too in the energy range including the C 1s threshold, where multiple desorption processes are possibly taking place.