Trimellitic anhydride is a 2-benzofuran compound having oxo groups at the 1- and 3-positions and a carboxy substituent at the 5-position; the cyclic anhydride formed from the carboxy groups at the 1- and 2-positions of trimellitic acid. It has a role as an epitope, an allergen and a hapten. It is a cyclic dicarboxylic anhydride, a dioxo monocarboxylic acid and a member of 2-benzofurans. It derives from a phthalic anhydride and a trimellitic acid.