In contrast to other low-pressure chemical vapor deposition (LPCVD) processes for SiO2 films, the deposition from tetraethoxysilane (TEOS) gas mixtures Seems to be essentially controlled by by-product inhibition. We report an experimental study aimed at the verification of such a mechanism. Ethanol was investigated as a possible inhibitor. The results cannot be explained by by-product inhibition alone. We therefore conclude that the by-product-inhibited deposition from the primary reactant TEOS occurs in par allel with a deposition from a second precursor formed in the gas phase (intermediate product).