In situ broadline 1H NMR has been used to assess the low temperature degradation of polyacrylonitrile (PAN) under a variety of atmospheres and temperatures. In combination with conventional analytical techniques (solid state 13C NMR and FTIR), the structural and conformational changes produced in the network have been correlated with its thermal behaviour. Degradation has also been shown to be both temperature and time dependent, irrespective of reaction environment. Moreover, it has been demonstrated why air is the preferred medium for stabilisation in the production of fibre precursors. Enhanced stability and resistance to char formation is observed in the air-modified polymer, which also becomes resistant to resoftening during low temperature carbonisation.