Deposition of ultrathin silicon dioxide films on Mo(100) substrates by pyrolysis of tetraethoxysilane (TEOS) vapor has been investigated at temperatures between 300 and 860 K with X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and infrared reflection−absorption spectroscopy (IRRAS). Up to temperatures of ∼600 K, TEOS adsorbs on the Mo surface forming an ethoxysilyl intermediate, whereas at higher temperatures SiO2 is formed during the initial exposure, as evidenced by both XPS and IRRAS data. Deposition of silicon dioxide is reaction-limited in the temperature range studied with roughly a monolayer forming on the surface at 860 K. Heating the TEOS-exposed Mo(100) surfaces to ∼1000 K yields ethylene as the predominant gas-phase decomposition product and improves both the stoichiometry and order of the films as indicated by an increase in the stretching frequency of the Si−O IRRAS peaks. A decrease in the amount of desorbed ethylene is observed as the deposition temperature increases from 300 to ∼600 K, and no significant desorption of any other decomposition products was detected at higher deposition temperatures. Carbon contamination is minimal in these SiO2 films.