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Lithium codoping has emerged as an effective strategy to enhance the light yield of oxide scintillators for radiation detection applications, but the understanding of the actual role played by Li + remains unclear. In this work, we comprehensively study the effects of Li codoping on optical and scintillation properties of Lu 2 SiO 5 :Ce (LSO:Ce) single crystals and reveal the critical role of site occupancy of Li. High-quality LSO:Ce single crystals codoped with 0.05, 0.1, and 0.3 at. % Li ions were grown by the Czochralski method. The optical absorption spectra confirm nonconversion of stable Ce 3+ to Ce 4+ in Li-codoped LSO:Ce regardless of the Li codoping concentration. The photoluminescence decay kinetics suggest an enhanced ionization of the excited 5d 1 state of Ce 3+ centers in highly codoped samples. A simultaneous improvement of scintillation light yield, decay time, and afterglow is achieved in LSO:Ce codoped with low concentrations of Li. The preferential occupation of Li at interstitial spaces and lutetium sites is proven to rely on its codoping concentration by using the 7 Li nuclear magnetic resonance technique. The concentration-dependent site occupancy of Li alters the defect structures of LSO:Ce, in particular resulting in a distinct change in the number of cerium spatially correlated oxygen vacancies confirmed by thermoluminescence and afterglow measurements.