A chemically defined compound has a single, specific molecular identity and a definite chemical formula—for example, mercuric chloride. Mixtures or preparations of mercury compounds that do not meet that single-identity standard fall under the 'not chemically defined' subheading (285290). The distinction matters because it determines the correct 6-digit subheading and can affect applicable duty rates.

EPA administers import restrictions on mercury and mercury compounds under the Toxic Substances Control Act (TSCA) and the Mercury Export Ban Act. Importers may need to file TSCA certifications and demonstrate that the compounds are not destined for prohibited uses. Consulting a licensed customs broker and reviewing EPA guidance before shipment is strongly advised.