Poly(aryl-ether-ether-ketone) (PEEK) has been heated at 400°C in both nitrogen and air. The extent of thermal degradation has been monitored by a variety of techniques including: solution viscometry, UV-visible spectroscopy, 13C high-resolution cross-polarization/magic angle spinning nuclear magnetic resonance and Fourier transform infrared spectroscopy. The results indicate, that in a nonoxidative environment such as nitrogen, PEEK is relatively stable at 400°C for periods up to 6 h. Under oxidative conditions chain scission reactions occur and crosslinks are formed causing a reduction in the crystallizability of the material and discoloration of the polymer. The spectroscopic evidence suggests that the chain cleavage occurs adjacent to the carbonyl functional groups. Subsequent hydrogen abstraction from the aromatic ring then leads to crosslinking between adjacent aryl rings.