Silicon nitride powders have been thermally oxidized between 700 and 1200 ΓC in a high-purity gas N 2 -20% O 2 environment. The powders were subsequently analyzed by x-ray photoelectron and Auger electron spectroscopies for evidence of oxynitride surface states. Measurements were made on the Si 2p, O 1s, N 1s, C 1s, F 1s and Si KLL transitions, the latter being obtained using bremsstrahlung radiation from the Mg x-ray source. As a function of increasing temperature the data show a clear progression of spectral binding energies and peak shapes that are indicative of more advanced surface oxidation. However, deΓnitive analysis of these data rests on the combined use of both Auger and photoelectron data to deΓne the oxidized surface states for a system that involves two electrically insulating end states : silicon nitride and silicon dioxide. Curve Γtting the Si 2p and Si KLL transitions as a function of oxidation, coupled with the use of Auger parameters for the starting silicon nitride and Γnal silicon dioxide, reveals no measurable evidence for an interphase oxynitride in the thin oxide scales of this study where the silicon nitride substrate is detectable. Possible incorrect assignment of oxynitride bonding, from shifted Si 2p states in the carbon referenced spectra, is attributable to band bending as the transition is made from incipient to fully formed silicon dioxide.