The thermal oxidation of Parylene N and Parylene C has been studied in connection with the use of these polymers in conservation. The results of previous investigations have been compared, confirmed and expanded. On the basis of measurements of degradation at temperatures between 125°C and 200°C, the useful lifetime of Parylene as a consolidant at 25°C (in the absence of light) is estimated as 2,200 years for N and 130,000 years for C. Incorporation of the antioxidant 4,4'-methylene-bis-(2,6-di-tert-butyl-phenol) increases the stability substantially but, in view of the very high stability of the pure film, its use in conservation is regarded as nonessential. The effect of light is not considered in this work.