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The previously developed TriA-X polyimide resin, consisting of pyromellitic dianhydride/2-phenyl-4,4′-diaminodiphenyl ether; 9,9-bis(4-aminophenyl)fluorene/4-phenylethynylphthalic anhydride (PEPA), was exposed for 3000 h to air at 180, 240, and 270°C or a vacuum environment at 270°C, in order to study its long-term thermal stability. The weight loss and dimensional changes were measured. Elemental analysis was also conducted in order to estimate the TriA-X degradation process. The TriA-X exhibited excellent thermal stability at 180°C and a very small degree of degradation at 240°C, with the degradation becoming significant at 270°C. The latter was primarily dominated by the oxidation of a chain-extension moiety formed by the cure reaction of the end-cap PEPA.