The present issue, Volume 2 of "Boron Compounds" 4th Supplement of the Gmelin Hand­ book, updates the previous issues by reporting the literature on boron-oxygen systems published up to 1988. For some important recent developments literature is covered through mid-1992; this concerns, for example, the compounds ~-Ba3[B306h and U[B305J which became of interest as materials with nonlinear optical properties. The volume directly com­ plements the earlier "Boron Compounds" 3rd Supplement Volume 2. In the original literature, alternative formulations are frequently used for the same com­ pound. This is especially true for many borates. Often, these species are neither completely heteropolar nor covalent, and an experimentally based decision has not been made. Hence, the use of brackets does not necessarily reflect a truly salt-like character. Volume 1 (systems with hydrogen and noble gases) of this particular supplement will be published subsequently, whereas Volume 3a (boron and nitrogen), Volume 3b (boron and nitrogen, boron and fluorine), and Volume 4 (boron compounds containing Cl, Br, I, S, Se, and Te, as well as a section containing carboranes) have already been published. All volumes of the 4th supplement will be augmented by a formula index. The IUPAC nomenclature is generally adhered to; thf means tetrahydrofuran; and occa­ sionally additional abbreviations for compounds are explained in the text. Positive signs for chemical shifts of the NMR signals indicates downfield shifts from the references, usually internal (CH3)4Si for olH and 013C with others being specified.