The thermal stability of poly(p-phenylenevinylene) (PPV), along with its ability to form flexible devices, has sparked a considerable amount of research in the area of polymer light emitting diodes (LEDs). The presence of carbonyl groups as defects in the PPV structure has been reported elsewhere.(1) An increase in carbonyl groups correlates well with a dramatic reduction in the photoluminescence and electroluminescence of this polymer. We have discovered that these carbonyl defects can be introduced thermally as well as photochemically. This paper reports the mechanisms for the thermal and. photochemical oxidation of PPV. Although PPV is thermally quite stable, its hydroxy containing defects present in the precursor polymer are very susceptible to reaction with traces of oxygen at the elevated temperatures used in thermal conversion, resulting in the formation of ketone based carbonyl impurities. In addition, pristine PPV is very unstable in the presence of light and oxygen. The photooxidation of PPV involves the cleavage of the vinyl double bond and the formation of terminal aldehyde groups. These results have important implications for the stability of this polymer in LEDs, where elevated temperatures or light create highly reactive charged species.