It has been demonstrated on the basis of absorption spectra measurements that the activation of glassy lithium tetraborate by terbium results in the formation of band of localized electronic states in the pseudo band gap, which correspond to the energy levels of rare earth element ions. As follows from the anal ysis of the X ray luminescence spectra, the activator ion plays the most important role in the emergence of luminescent properties of glassy lithium tetraborate. It has been established that X ray luminescence in glassy lithium tetraborate activated by terbium is mostly realized due to transitions from excited 5 D 3 and 5 D 4 states to the levels of spin orbital splitting of the main 7 F J state of Tb 3+ ion.