An investigation of the photoluminescence properties of divalent thulium generated by X-ray irradiation (X-irradiation) of nanocrystalline BaFCl:Tm3+ (250 ± 50 ppm) is reported. The X-irradiated samples show typical Tm2+ f-f luminescence with an excited state lifetime τ = 0.98 ± 0.05 ms in the near-infrared (∼1140 nm), but no d-f luminescence is observed for the BaFCl host material within the visible range. The Tm2+ ions are relatively stable under dim room light but are photoionized rapidly by sunlight or by fluorescent tube lighting. To study the X-ray storage mechanism, photoluminescence intensities of both Tm3+ and Tm2+ transitions were measured, and the reverse photoionization of Tm2+ was investigated as a function of the laser power density and the initial radiation dose. The photoionization data was then modeled by employing equations based on dispersive first order kinetics using a standard Γ distribution function for the separation between the reduced thulium ions and the hole traps. In accord with previous reports, the hole traps are most likely oxide impurities such as O2-, and the average of the separation is found to be ∼6-7 Å, i.e., a few interionic spacings.