Thermal oxidation of (100) and (111) silicon (Si) in a nitrous oxide (N2O) ambient has been studied. The oxidation rate follows a parabolic to linear behavior. The activation energies in the parabolic regime were found to be 2.02 and 1.54 eV/molecule for (100) and (111) Si, respectively. The activation energy for the linear rate constant is estimated to be 1.61 and 1.37 eV/molecule for (100) and (111) Si, respectively. The limiting mechanism for the parabolic regime is attributed to the diffusion of oxidant through a surface oxynitride layer. The gradual shift to linear behavior is unusual and is in direct contrast with the Grove–Deal model [J. Appl. Phys.36, 3770 (1965)]. Finally, secondary ion mass spectrometry shows a temperature dependent distribution and concentration of nitrogen at the interface. ©1995 American Institute of Physics.