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Trivalent cerium ions form the luminescence centers in several important families of scintillation materials including the rare earth oxyorthosilicates, pyrosilicates, and aluminates. When comparing the experimentally determined scintillation properties of cerium-doped scintillators to theoretical models of scintillation mechanisms, there is often speculation regarding the fraction of the total cerium that exists in the radiative trivalent charge state (Ce3+) rather than the nonradiative tetravalent state (Ce4+). Until now, however, no technique has been developed to quantitatively measure both Ce3+ and Ce4+ in single crystal scintillators . We report here for the first time direct measurements of Ce3+ and Ce4+ in Lu2SiO5:Ce single crystals. Synchrotron radiation was used to measure the X-ray absorption on the M4 and M5 edges of Ce, and the results were compared to model samples of Ce3+ (Ce2O3) and Ce4+ (CeO2) which provided clear signatures of the two charge states. The spectra were obtained with a high-resolution superconducting tunnel junction spectrometer on beamline 4.0.2 at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The results clearly show 100% Ce3+, independent of light yield and sample coloration. Therefore, energy migration to the luminescence centers appears to be the determining factor in the scintillation efficiency of these samples, rather than variations in the Ce3+/Ce4+ ratio.