Mono-doped Ce:LYSO (Lu 1.80 Y 0.2 SiO 5 :Ce) and co-doped Ca,Ce:LYSO and Mg,Ce:LYSO single crystals were prepared by the Czochralski technique. Effects of the X-ray irradiation on the emission spectra and on the light yield are reported. A decrease of the longest wavelength emission (at about 500nm) and a light yield increase are observed after 1h under X-ray irradiation. As electron paramagnetic resonance (EPR) evidenced no change in the two cerium sites occupancy along with the irradiation time a mechanism based on the evolution of the oxygen vacancies as neighbors of Ce 3+ ions is proposed.