The Gmelin Institute presents the Third Supplement of the Gmelin Formula Index (GFI) which covers 95 new volumes of the Gmelin Handbook of Inorganic and Organometallic Chemistry published between 1988 and 1992. With this extension the GFI contains more than 400 000 elements, compounds, systems, and other types described in 620 volumes. The Third Supplement includes more than 65 000 compounds sorted according to the empirical formula and characterized by a linearized structural formula. As in previous sections of the index, compounds are further characterized by keywords if more information is available in the Handbook. Systems, mixed crystals, solutions, glasses, isotopes, isotopomers (also highly enriched), and other compound classes are separately listed. The GFI provides fast access to any inorganic or organometallic compound and the relevant page numbers in the Handbook. It is one of the means to facilitate searching for the large amount of available information on the chemical and physical properties of compounds. The Third Supplement will be published in 6 volumes.