The results of the optical polarization study of domain structure and impedance spectroscopy measurements of conductivity of CsDSO4 single crystals doped with a small amount of PO4 anions are presented. The activation energy of CsDSO4 crystals doped with 2-5 mol% PO4 at superprotonic phase is 0.30-0.33 eV and conductivity 6 x 10(-3) Omega(-1) cm(-1). The temperature variation of conductivity appears as a two-step process, the bulk conductivity decreases stepwise to 10(-5)-10(-6) Omega(-1) cm(-1), whereas the activation energy rises to 0.7-0.8 eV.The electric anomalies which were observed near the superprotonic phase transition at T-s - 4 K are accompanied by the appearance of a new domain pattern and associated with an intermediate phase. The intermediate phase in phosphate-doped CsDSO4 crystals disappears with the increasing phosphate concentration. Our observation of the domain structure allows us to propose a cubic hypothetical prototype phase.