The effects of adding NO2, NO, and NH3 on the chemical vapor deposition (CVD) of SiO2 from SiH4/O-2 were studied using a hot-wall type tubular reactor operated at a temperature of 873 K. Without additives, rough films with poor step coverage were obtained. Adding NH3 and NO2 resulted in clear films with good step coverage. However, adding NO did not improve the quality of the film or of the step coverage. Numerical simulations of the gas-phase elementary reaction kinetics of the SiH4/O-2 reaction system were made for the same conditions as the experiments. The simulations show rapid conversion (i.e., within 0.007 s) of SiH4 into SiO2, resulting in high concentrations of SiO2, which might form clusters that deposit to form rough films. Numerical simulations including NH3, NO, and NO2 showed that NH3 and NO2 reduced the gas-phase SiH4 reaction rate and that NO did not affect it. These numerical results agree with our experimental results and show that simulations of gas-phase, elementary chemical reactions are sufficiently accurate to reproduce the behavior of some aspects of CVD processes.