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The thermal stability in air and nitrogen of an oxynitride NaPON glass with high nitrogen content (N/P = 0.5) has been investigated with regards to its structural evolution with temperature. The glass transition temperature (T g) of the powdered glass is found to decrease upon oxidation, especially when the treatment temperature (T a) is larger than the T g of the original oxynitride glass. Upon isothermal oxidation, crystalline metaphosphate forms at the interface of the oxide layer as the dominant phase in both the powder and bulk samples, as detected by Raman spectroscopy and X-ray diffraction. A new deconvolution scheme of Raman spectra is proposed, involving a structural model proposed to account for the in situ high temperature changes of the local structural groups. A distinction is made between different oxynitride Q n(P,N) tetrahedral sites, and two separate bands related to tri-coordinated nitrogen speciation (N t) are distinguished in the oxidized NaPON glass. N t groups are connected to either one oxygen or one nitrogen, resulting in two separate Raman bands. The position and area of these N t-related peaks exhibit an opposite trend with temperature in air and N 2. Furthermore, the Raman results imply a thermally driven depolymerization of the oxynitride sub-structure, which could involve a nano-scale phase separation of the nitrogen-involved structure network. In terms of technological applications, this work suggests that the oxynitride glasses should be used in the temperature range up to the glass transition temperature, above which the structural stability is lost.