We present the research results on transient optical absorption (TOA) and luminescence for lithium-gadolinium orthoborate Li 6GdB 3O 9 (LGBO) in the visible and ultraviolet spectral regions. According to absorption optical spectroscopy with nanosecond time resolution studies the LGBO TOA is caused by optical transitions in hole centers. The optical density relaxation kinetics is governed by interdefect tunneling recombination between these centers and neutral lithium atoms acting as electronic Li 0 centers. At 290 K Li 0 centers are involved in thermally stimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. We suppose that diffusion-limited annihilation of lithium interstitials with vacancies is responsible for the slow TOA decay kinetics component with characteristic times ranging from a few milliseconds to seconds. The mechanisms of defect creation and short-living Frenkel pairs relaxation in the LGBO cation sublattice were analyzed.