The authors have investigated the synchrotron radiation (SR) induced chemical reactions of condensed layers of silicon alkoxides, tetramethoxysilane Si(OCH3)(4) and tetraethoxysilane Si(OC2H5)(4), adsorbed on a Si substrate at 80 K. Photon stimulated desorption measurements show that irradiation with SR in the vacuum-ultraviolet region induces the desorption of hydrogen, hydrocarbon, and carbon monoxide from the condensed layer, indicating that the radiation decomposes the alkoxyl group in the silicon alloxides. Infrared absorption and photoemission data demonstrate that Si-containing fragments produced by the decomposition of silicon alkoxide molecules are polymerized to form silicon oxide. The experimental results show the possibility of deposition silicon oxide film from silicon alkoxides using intense SR in the VUV region.