The results of thermal oxidation studies in N2O on P (100) silicon substrates having initial dry oxygen oxide thicknesses of up to 145 A, indicate that the growth characteristics of ultra-thin SiO2 films show no signs of dielectric thickness saturation. For total dielectric thicknesses as large as 160 λ, N2O oxidation rate is generally linear, and significantly smaller than the corresponding rate in dry oxygen. This linear regime has been modeled by a linear rate constant with an activation energy of 1.0 eV. Prior to the linear regime, a short accelerated growth phase has been observed. As the initial oxide thickness on the substrate is increased, the results indicate that the initial accelerated growth characteristics become less and less pronounced. An empirical model, based on the gradual accumulation of rate retarding nitrogen species at the Si/SiO2 interface, has been developed and added to a modified accelerated growth rate model, to successfully simulate this phase. The results are consistent with AES data which show that the higher the initial oxide thickness on the substrates, the greater the nitrogen peak concentration at the Si/SiO2 interface for a given N2O exposure time.