The results of an optical polarization study of superprotonic CsHSO4 and CsDSO4 crystals in the vicinity of phase transitions are presented. Our observations allow us to state that the temperature of III-II phase transition in ’fresh’, high quality crystals is 373 K. The temperature of the phase transition of the specimens that have experienced the III-II phase transition earlier is reduced to 330 K. It is established that changes of the domain structure in CsDSO4 crystals are observed on both increasing and decreasing temperature at 3 K away from the temperature of the superprotonic phase transition. The appearance of the new structure is accompanied by a gradual increase in the conductivity by 1.5-2 order of magnitude and then the conductivity increases abruptly by another two orders of magnitude at the temperature of the superprotonic phase transition.