Niobium-codoped Ba[sub 5]SiO[sub 4]Br[sub 6]:Eu[sup 2+] acts as a photostimulable X-ray storage phosphor. The phosphor is also thermally stimulable. After X-ray irradiation, the EPR spectrum of Nb[sup 4+] is observed. UV irradiation at wavelengths between 250 and 340 nm has the same effect. It is concluded that an electron is trapped at the niobate group. The phosphor is optically stimulable between 300 and 650 nm. The corresponding absorption band is tentatively ascribed to a NbO[sub 4][sup 4[minus]] group. In the region between 300 and 340 nm, a competition between photoionization and stimulation takes place, resulting in characteristic thermoluminescence behavior. 27 refs., 3 figs., 1 tab.