Rare earth (RE)-activated orthophosphates were thoroughly investigated for their luminescent properties years ago. However, it is only recently as the singly-activated LuPO4:Eu was reported to show efficient thermoluminescence when exposed to ionizing radiation. In this presentation we shall compare and discuss X-ray induced profound changes in spectroscopic properties of the two high-temperature sintered ceramics: LuPO4:Eu and LuPO4:Sm. After exposure to ionizing radiation, both Eu- and Sm-activated LuPO4 ceramics show an efficient orange-red thermoluminescence (TL) with a maximum signal around 200°C and covering ~130-280 °C range of temperatures (see Fig. 1). The mechanism of the energy storage involves the Eu3+→Eu2+ or Sm3+→Sm2+ transformation due to trapping an electron from the conduction band. Spectroscopic effects related to the generation of Eu2+ or Sm2+will be presented and compared for both compositions. Fig. 1. Temerature- and wavelength-resolved thermoluminescence of LuPO4:Eu and LuPO4:Sm sintered ceramics. Figure 1