Photon-stimulated ion desorption from Si(OCH3)(4) [tetramethoxysilane (TMOS)] adsorbed on Si(100) following silicon K-edge excitation has been investigated using monochromatized synchrotron radiation. In order to separate the photochemical process induced by the substrate excitation from that induced by adsorbate excitation, we have prepared well-controlled monolayer, submonolayer and multilayer coverages of TMOS on Si(100). In the x-ray absorption near-edge structure (XANES) spectrum for a multilayer of TMOS, the peak energy of the resonant excitation from the Si Is to the unoccupied sigma* orbitals was shifted to higher energy by 6 eV than that for the silicon substrate. For a thin layer such as the monolayer and submonolayer, the XANES spectra taken by total electron yields were almost the same as those of the silicon substrate. However, the photon-energy dependencies of the CH3+ and Si(OCH3)(3)(+) yields for the monolayer sample did not follow the XANES spectra of silicon but they followed those for multilayered TMOS. This result indicates that the secondary electrons produced in the silicon substrate scarcely contribute to the fragment-ion desorption for thin layers. It was concluded that the core-to-valence resonant excitation in monolayered TMOS is the main trigger of the C-O bond scission and the fragment-ion desorption.