Abstract: Data on the synthesis of double borates crystallizing in the BaO–Lu2O3–B2O3 system and Eu3+-activated phosphors based on them, as well as on the finding of correlations between their chemical composition, crystal structure, and thermal and optical properties are presented. Based on the results of the investigation of all currently known double Ba–Lu borates, it is shown that search for novel compounds in this system, as well as the development of optical materials on their basis, is a promising direction in the field of creating new functional materials for LED applications.