The integration of high-kappa dielectrics, such as Ta2O5, in metal-insulator-semiconductor structures requires the careful control of interfacial silicon-oxide growth to preserve electrical properties such as capacitance. This has led to the introduction of silicon nitride films as oxidation resistant barriers between the dielectric and silicon. In this study, we examine the oxidation resistance of ultrathin (<2 nm) nitride films produced from NH3 exposure in ultrahigh vacuum are examined with in situ scanning tunneling microscopy and x-ray photoelectron spectroscopy. We find that ordered nitrides, grown at 950-1000 degrees C, are resistant to oxidation up to 600 degrees C for subatmospheric oxygen exposures. Amorphous nitrides, grown at 600 degrees C, exhibit similar oxidation resistance behavior.