The reactive adsorption and decomposition of tetraethoxysilane is compared on Si(100) and SiO2 surfaces. The adsorption and decomposition behavior is compared to that observed for ethanol adsorption on both these surfaces. Tetraethoxysilane and ethanol both decompose to produce ethylene and hydrogen on Si(100). Ethylene desorption is also observed from decomposition of these molecules on SiO2. Ethanol adsorption on SiO2 is shown to model the chemistry of tetraethoxysilane on this surface. However, ethanol is significantly more reactive than tetraethoxysilane on the clean Si(100) surface. The decomposition is shown to be reaction limited, and distinct from that of adsorbed ethylene. The adsorbed species produced from adsorption of tetraethoxysilane on SiO2 at 450 K is shown to be a mixture of di‐ and triethoxysiloxanes.