In order to determine the features of polarization and structural properties of perfluorosulfone membranes after their modification with Tb, MF-4SK membranes have been prepared and studied. The sorption properties of membranes were investigated by recording their optical absorption and transmission spectra in the visible and UV ranges in dynamics, as well as infrared reflection spectra. The temperature-frequency dependences of the main dielectric parameters of the objects under study, their spectra of complex conductivity were recorded and analyzed. As a result, it was found that the specific conductivity of terbium-modified perfluorosulfone membranes obeyed a power law. The presence of critical temperature of the system’s transition from semiconductor conductivity to metallic one was revealed. A scheme for the ion-exchange mechanism of the sorption fixation of terbium ions in the membrane was put forward.