The emission and excitation spectra of single crystals of CaGa2S4 doped with Ce3+, Pr3+, Tb3+, Er3+, and Tm3+ were measured with the aim to determine the location of the lanthanide energy levels relative to the valence and conduction band of the host crystal. By combining energies known for charge transfer bands involving trivalent lanthanides with systematic behavior in 4f-5d transition energies of divalent lanthanides, the level positions of 4f and 5d states for each divalent lanthanide can be drawn relative to the host levels. With similar techniques combined with results on the thermal quenching behavior of Ce3+ emission, the levels for the trivalent lanthanides were constructed. We demonstrate consistency between the proposed level scheme and the spectroscopic information presented here and elsewhere. The scintillation properties of Ce3+-doped CaGa2S4 are addressed also. (C) 2004 The Electrochemical Society.