The decomposition of tetraethoxysilane (TEOS) on a thermally grown SiO2 surface has been studied using molecular beam scattering, with mass spectrometric detection and time of flight techniques. Decomposition was characterized over a range of temperatures from ambient to 1100 K using a mass spectrometer having a line of sight to the sample surface. Time of flight measurements were used to characterize parent and product species and to determine surface residence time. The initial step in the decomposition process at low temperatures is the loss of one ethoxy ligand to produce triethoxysilane, ethylene and possibly water as gas phase products. At temperatures above 850 K there is a change in the rate limiting process and an abrupt increase in the decomposition probability. Observed kinetics are explained in terms or an adsorbed molecular precursor mechanism.