Abstract-The nonlinear dependence of the relative light output on the energy deposited in single crystal scintillation materials YAlO 3 :Ce (YAP:Ce), Y 2 SiO 5 :Ce (YSO:Ce), and YPO 4 :Ce (YPO:Ce) has been studied. The investigations have been conducted under quasi monochromatic X ray excitation in the energy range of 9.5-100 keV. In addition to the standard technique for measuring the nonproportional scintillator response based on the dependence of the full energy peak position on the energy of incident radiation, a method is proposed for measuring the light output by X ray fluorescence peaks. Using this method for YAP:Ce, it is pos sible to investigate the nonlinear dependence of the light output on the photon energy in the energy range of 2-40 keV. Along with this method, the K dip spectroscopy method has been proposed and tested by measur ing the dependence of the relative light output on the electron energy in the range of 0.1-80.0 keV. The pro cesses resulting in the loss of the scintillation material efficiency at a high ionization density are considered.