Langmuir-Blodgett films prepared from diacid-terminated poly(dimethylsiloxane) can be transformed to SiOx ultrathin films (t = 14 Angstrom) by exposure to UV-ozone at room temperature and atmospheric pressure. The kinetics of the reaction is followed by grazing-incidence infrared spectroscopy and the film composition is determined by angle-resolved X-ray photoelectron spectroscopy. The results show that UV-ozone treatment oxidizes 90% of the methyl side groups to hydroxyl and bridging oxygen species. This technique represents a novel low temperature route for the preparation of ultrathin SiOx films from polysiloxane starting materials.