Effects of an oxygen flow rate on film formation and nanometre-sized particle generation in the gas phase were examined simultaneously in an atmospheric-pressure chemical vapour deposition reactor using tetraethylorthosilicate (TEOS). The critical temperature for particle generation decreased rapidly to 340 degrees C from 740 degrees C with increasing oxygen flow rate, but it decreased slightly to 600 degrees C from 700 degrees C for film formation. There were no conditions where film was deposited without particle generation in a TEOS/O-2 system. The nanometre-sized particles generated in the systems were amorphous and non-spherical, and their size distributions were polydisperse. The Fourier transform infrared (FT-IR) and thermal desorption (TDS) spectra of the particles were not affected by oxygen flow rate, and showed that the particles contained a small amount of an ethoxy group and a relatively large amount of a hydroxyl group. It was found from comparisons between FT-IR and TDS spectra of particles and films that the SiO2 films were formed by beta-elimination reactions, where C2H4 and H2O are released from the ethoxy group.