Perfluoroelastomer (e.g., Kalrez(R), Chemraz(R), etc.), fluorelastomer (e.g., Viton(R), etc.), and silicone parts are widely used in sealing applications for semiconductor wafer processing equipment. More specifically, they are often used as o-ring seals in dry chemical process equipment (e.g., plasma etchers, ashers, diffusion furnaces, chemical vapor deposition, low pressure chemical vapor deposition, rapid thermal processing, and lamp anneal, etc.). Many of these seals are required to function at process temperatures ranging from 200-300 degrees C and in some cases higher. The ability of an elastomer to resist thermal degradation has a,significant impact on its ability to function effectively as a seal over time. This article evaluates and compares the relative long term sealing performance of perfluoroelastomers having different crosslinking chemistries and other typically specified high performance elastomers. Long-term compression set and seal force retention data are presented, including a discussion of the different test methods and their relative value in predicting seal performance at elevated temperatures. In addition, air heat aging data for other typically measured physical properties (i.e., shore A hardness, 100% modulus, tensile strength at break, elongation at break, percent weight loss) are also presented. and discussed. Results indicate perfluoroelastomers seals have the best long-term compression set characteristics land retain the highest percentage of their original. sealing force over time. In addition, data shows perfluoroelastomers having an organometallic cross-linking system are the most thermally stable and exhibit the best long term seal performance at elevated temperatures versus those having an organic-peroxide cross-linking system. Finally, because the test conditions best reflect static o-ring seal service conditions, analysis of seal force retention-test results provide a more realistic comparison of thermal stability and its relative impact on sealing performance.