The chemical reaction mechanism and fine particles generation during atmospheric pressure chemical vapor deposition with the tetraethylorthosilicate/ozone system were studied and theoretically for a flow-type vertical-tube reactor. For the thermal decomposition of ozone, numerical simulations obtained using reaction rate constants for a batch explained experimental results well. The activation energy of chemical reaction between TEOS and the oxygen radical, O ., was evaluated from the particle generation rate (2.18 x 10(5) J/mol). Particle population balance equations, based on the simplified reaction coagulation model, were coupled with equations for the thermal decomposition of ozone and chemical transformations of TEOS and solved numerically. Calculation results for particle number concentration and size agreed with experimental results for particle number concentration and size agreed with experimental results fro temperatures below 430 K. For temperatures above 430 K measured concentrations agreed well with calculated values, but measured sizes were significantly smaller than calculated values, but measured sizes were significantly smaller than calculated sizes. Thermal desorption spectra from particles generated at 423 K revealed a peak from C2H4 at similar to 800 K, presumably from ethoxy groups. This emission explains, at least in part, the observed decrease in particle size at high temperatures.