Ensuring the thermal reliability of luminescent materials is a key requirement for next-generation lighting, display, and sensing technologies. The intricate interplay of thermal crossover and thermal ionization in lanthanide-doped phosphors often obscures their individual contributions. We present a frequency-domain photoluminescence analysis that disentangles these competing mechanisms. Using single crystals of SrAl2O4:Eu2+,Dy3+ (SAO:Eu,Dy) and (Gd0.33Y0.67)3Al2.4Ga2.6O12:Ce3+,Cr3+ (GYAGG:Ce,Cr) as model systems, we extract temperature-dependent trapping efficiencies and decay rates by analyzing the phase and amplitude response of luminescence under modulated excitation. Our approach reveals distinct signatures of thermal ionization and enables the direct quantification of ionization barriers and crossover rates. We demonstrate that SAO:Eu,Dy exhibits dominant trapping behavior with high ionization efficiency, while GYAGG:Ce,Cr shows significant competition between ionization and crossover. This method provides a powerful framework for resolving overlapping quenching pathways and offers new insights for the design of thermally robust luminescent materials.