Crystals of Ba 7 (BO 3 ) 4-y F 2+3y and Ba 4-x Sr 3+x (BO 3 ) 4-y F 2+3y solid solutions display a phenomenon that has never been described for borates before -X-ray irradiation induces strong absorption in the visible range and, as the result, coloring of the crystals up to dark purple. The induced absorption can be removed by illumination with a wavelength of 300−400 nm. Owing to this phenomenon these crystals are promising sensor applicable to fixing X-ray images. With the use of electron spin resonance and optical spectroscopy it has been found that the formation of color centers is connected with the [(BO 3 )F] 4-and [F 4 ] 4-tetrahedral groups, which are the distinguishing feature of Ba 7 (BO 3 ) 4-y F 2+3y and Ba 4-x Sr 3+x (BO 3 ) 4-y F 2+3y structure. Partial replacement of Ba 2+ to Sr 2+ in the crystal lattice of Ba 4-x Sr 3+x (BO 3 ) 4-y F 2+3y leads to the formation of the stable electron-hole pairs under X-ray irradiation. The opportunity to vary the cationic and anionic sublattice composition within the same crystal structure becomes a prospective tool to vary physical and chemical properties of the phosphors.