The kinetics of SiO2 deposition by thermal decomposition of tetra-alkoxysilanes at pressures of 1.0 Torr or less and temperatures from 690-810 degrees C are investigated using a cold-wall reactor capable of in situ growth rate measurement. It is found that the deposition rate is almost independent of the total flow rate from 20-100 seem, indicating that the deposition is kinetic-controlled. The data for all of the TEOS experiments can be correlated very well over the entire range of temperature and pressure by a model in which TEOS adsorbs and dissociates on the surface according to a Langmuir-Hinshelwood mechanism. The resulting rate expression for TEOS involves two temperature dependent coefficients which were determined by using the experimental data. The deposition kinetics of other tetra-alkoxysilanes including tetramethoxysilane (TMOS), and tetrabutoxysilane (TBOS) are compared with TEOS. Under the same experimental conditions, TEOS provides the highest deposition rate and TMOS has the highest activation energy.