The crystal structure of the thallium selenate tellurate Tl(2)SeO(4)center dot Te(OH)(6) (TlSeTe) was determined by Xray diffraction method. The title compound crystallizes in the monoclinic system with P2(1)/c space group. The following parameters are: a = 12.358(3) angstrom: b = 7.231(1) angstrom: c = 11.986(2) angstrom; beta = 111.092(2)degrees; Z = 4. The structure can be regarded as being built of isolated TeO(6) octahedra and SeO(4) tetrahedra. The Tl(+) cations are intercalated between these kinds of polyhedra. The main feature of this structure is the coexistence of two different and independent anions (SeO(4)(2-) and TeO(6)(6-)) in the same unit cell. The structure is stable due to O-H center dot center dot center dot O hydrogen bonds which link tetrahedral and octahedral groups. Crystals of Tl(2)SeO(4)center dot Te(OH)(6) undergo three endothermal transitions at 373, 395 and 437 K. These transitions are detected by DSC and analyzed by dielectric measurements with impedance spectroscopy. The evolution of conductivity versus temperature showed the presence of a protonic conduction phase transition at 437 K. The phase transition at 373 K can be related to a structural phase transition, whereas the one at 395 K is ascribed as likely due to a ferroelectric-paraelectric phase transition. (C) 2009 Elsevier Ltd. All rights reserved.