Single crystals of K2SeO4. Te(OH)(6), (KTSe) were obtained by slow evaporation at 300 K. The differential scanning calorimetry diagram showed three anomalies at 433, 480 and 495 K, respectively. The dielectric constant evolution as a function of frequency and temperature revealed the first two anomalies. The peak at 433 K was attributed to the ferroelectric-paraelectric phase transition and the second one to the superionic phase transition. X-ray powder diffraction data confirmed that the third anomaly at 495 K characterizes the decomposition before melting of the material. (C) 2000 Elsevier Science BN. All rights reserved.