Temperature variations of Raman internal modes in the CsDSO 4 single crystal were studied in the vicinity of superionic phase transition. Anomalous behaviour was observed 3K below the superionic phase transition temperature T s . Within the same temperature range, new ferroelastic domains with (101) walls were observed in the (010) surface of the crystal. The results point to an existence either of a heterophase or a new phase in CsDSO 4 , in the temperature range T s -3K.