Abstract
The state-of-the-art in boron chemistry, the characteristic features of boron compounds and crystalline phases based on them, the chemical bonds in these compounds and their electronic structures are considered. The physical principles of X-ray spectroscopy and the results it provided on the chemical structures of boron compounds and the nature of chemical bonds in them are discussed. Data for X-ray photoelectron and ultraviolet photoelectron spectroscopies for various elemental boron modifications, boron oxide phases, borides, nitrides, halides, derivatives of boron hydrides and carboranes, as well as metalloboranes are compared.