We have studied the deposition of silicon dioxide by thermal chemical vapor deposition from tetraethylorthosilicate (TEOS) and ozone, using a unique linear gas injection configuration and operating pressures of 10 Tone to atmosphere (approximate to 760 Torr). We have compared the experimental results with simulations incorporating finite-difference estimation of fluid flow fields, temperature distribution, thermally initiated decomposition of ozone, and reaction of TEOS and ozone at the surface and in the gas phase. We have demonstrated that a parasitic gas-phase reaction, which consumes TEOS without contributing to deposition on the substrate, is necessary to explain correctly results obtained at high pressures, and that at pressures of less than 200 Torr, this reaction simply contributes to the total deposited film. We propose a general scheme for surface reactions, which accounts for the discrepancies between the simple gas-phase reaction model and the observed other temperature dependence, and allows us to incorporate aspects of the behavior of other organosilane precursors; we suggest explanations for observed variations in film properties with deposition rate, conditions, and substrate surfaces.