Abstract <p>The incorporation of Li⁺ ions into crystalline materials enables the controlled modification of their physical properties; however, the efficiency of this process is determined by the mechanism of incorporation and charge compensation, which depend on many factors and are not always straightforward. In the present study, thulium aluminum garnet (Tm₃Al₅O₁₂), also known as TmAG, was chosen as a model system. Single crystals of TmAG, TmAG:Li, and TmAG:Ce, Li with Li concentrations of 50 and 150 ppm were grown by the Bridgman method, and polycrystalline samples of TmAG and TmAG:Li with Li concentrations up to 600 ppm were synthesized using solid-state reaction and melt-quenching techniques. The unit cell parameters of the samples were measured and compared with calculated values obtained from theoretical models. Absorption spectra of TmAG:Li and TmAG:Ce, Li single crystals were recorded before and after γ-irradiation. Experimental results have shown that at least a portion of the Li⁺ ions in TmAG occupy crystallographic lattice sites, like the behavior of Li⁺ ions in lutetium alumina garnet. It is suggested that Li⁺ ions are most likely to replace Al³⁺ ions.</p>