Barium orthophosphate doped with Ce3+, Pr3+, Gd3+, or Tb3+ acts as an efficient X-ray storage phosphor. The four compositions have different thermal stimulation properties; the thermally stimulated luminescence (TSL) occurs at higher temperatures than in Ba3(PO4)2:La3+. Similar to this latter compound, atomic hydrogen is present after irradiation in Ba3(PO4)2:Ce3+ and Ba3(PO4)2:Gd3+, but its electron paramagnetic resonance (EPR) signal disappears before the TSL takes place. In Ba3(PO4)2:Pr3+ and Ba3(PO4)2:Tb3+, no H0 is found. In the four cases studied, codoping with Eu2+ changes the TSL properties. In view of these results it is concluded that the storage mechanism is different for barium phosphate doped with luminescent trivalent rare earth ions on the one hand, and barium phosphate with La3+ or Y3+ on the other.