The European Commission maintains European Customs Inventory of Chemical Substances (ECICS) database, which, contains Harmonized Tariff information on various chemical substances. European Commission, as the United States, is obliged to apply “General Rules for the interpretation of the Harmonized System and all the Section, Chapter and Subheading Notes, and shall not modify the scope of the Sections, Chapters, headings or subheadings of the Harmonized System.” See International Convention on the Harmonized System, Article 3(1)(a)(ii). Therefore, ECICS database, is highly persuasive source of information for chemical substances up to six digit level.
The EU tariff website has an HTS look up feature for chemicals and metals based on CAS#. Some CBP employees use this as a source for additional guidance (to six digit level). http://ec.europa.eu/taxation_customs/dds/cgi-bin/eciquer?Periodic=0&Lang=EN