This study examines the light yield non-proportionality of Lu 1.8 Y 0.2 SiO 5 :Ce 3+ (LYSO:Ce), Lu 3 Al 5 O 12 :Ce 3+ (LuAG:Ce), (Gd,Lu) 3 (Al,Ga) 5 O 12 :Ce 3+ (GLuGAG:Ce), and LaBr 3 :Ce 3+ (LaBr 3 :Ce) to low energy electrons. The intrinsic radiation of these scintillators makes the study of the light yield, implementing the conventional Compton coincidence technique, a difficult task. In this work, a modified Compton spectrometer based on a triple-coincidence condition with three detectors was developed to reject contributions from the natural radioactivity of 176 Lu and 138 La. The experimental setup detects the annihilation photon pair, produced by an 18 F positron source, in coincidence with a Compton-scattered photon at variable scattering angles within a coincidence time window. The relative light yield and energy resolution of the crystals to electrons were measured over the 43–317 keV energy range. In addition, the light yield and energy resolution were also measured using gamma rays over the 32–511 keV energy range. The results revealed a proportional response of all the crystals to both electrons and photons above 100 keV. LuAG:Ce and GLuGAG:Ce showed a more proportional light yield to electrons compared to LYSO:Ce and LaBr 3 :Ce. LaBr 3 displayed the best energy resolution owing to its highest light yield. These findings agree with previous publications using the traditional Compton coincidence technique.