Changes in the luminescence of poly(ethylene terephthalate) (PET) degraded in the absence and presence of oxygen at melt temperatures has been investigated. On oxidative degradation monomer fluorescence is reduced, with a concurrent increase in dimer fluorescence at long wavelengths. This is consistent with main chain breakdown and aggregation of the terephthalate fragments produced. Longer wavelength fluorescent species have been assigned to hydroxylation of the aromatic rings of PET. Further oxidation of PET degraded in air elicits changes in fluorescence and phosphorescence which have been associated with oxidation of hydroxylated units which exist in equilibrium with quinones by a redox mechanism. Phosphorescence studies on extracts from heavily oxidized PET have indicated the presence of stilbene quinone type impurities. The absence of such changes in the luminescence of PET degraded in nitrogen indicates hydroxylated, quinone and stilbene quinone species arise primarily from oxidation reactions. These species ale suggested to contribute to the yellowing of PET on degradation.