The adsorption and reaction of tetraethoxysilane (TEOS) with hydroxylated SiO2 has been studied over the range of 100–450 K using transmission infrared spectroscopy. At 100 K, TEOS [Si(OC2H5)4] condenses on SiO2. Upon warming in vacuum, some of the condensed phase TEOS desorbs molecularly while a significant portion of the layer enters into a physisorption state. The physisorption state maximizes near 250 K, with strictly molecular desorption occurring upon warming to higher temperatures. When exposure occurs at 450 K, Si(OC2H5)4 reacts to form adsorbed siloxanes, thought to be a mixture of (SiO)2Si(OC2H5)2 and (SiO)Si(OC2H5)3. The adsorbed di‐ and triethoxysiloxanes decompose completely on heating in vacuum to 900 K. The chemistry of TEOS on SiO2 has been modeled using ethanol adsorption. Exposure of SiO2 to ethanol at 450 K leads to the formation of an adsorbed ethoxide species. Ethanol is shown to spectroscopically and chemically model the surface bound siloxanes produced upon reaction of Si(OC2H5)4 with hydroxylated SiO2 at 450 K. The vibrational spectrum of adsorbed ethoxide (SiOC2H5) is very similar to that of the adsorbed siloxanes produced from the adsorption of Si(OC2H5)4. The ethoxy modes are assigned through comparison of C2H5OH and CH3CD2OH adsorption. The temperature dependence of the decomposition of the adsorbed ethoxide is similar to that of the TEOS derived siloxanes. Decomposition of the adsorbed siloxanes is shown to evolve primarily ethylene.