The reactions of tetraethoxysilane (TEOS) on oxidized Si(100) surfaces were studied using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy-loss spectroscopy (HREELS). The surface reaction pathways followed by TEOS were found to depend on the thickness of the silica layer. Changes in the observed reactivity with thickness were correlated to the relative populations of the various oxidation states of silicon present in the surface layer. The structure of the transition region between a Si(100) substrate and a SiO2 film deposited using TEOS was found to be similar to that produced by thermal oxidation using oxygen.