Ongoing studies of the KHSeO4-KH2PO4 system aiming at developing novel proton conducting solids resulted in the new compound K-2(HSeO4)(1.5)(H2PO4)(0.5) (dipotassium hydrogenselenate dihydrogenphosphate). The crystals were prepared by a slow evaporation of an aqueous solution at room temperature. The structural properties of the crystals were characterized by single-crystal X-ray analysis: K2(HSeO4)1.5(H2PO4)0.5 (denoted KHSeP) crystallizes in the space group P (1) over bar with the lattice parameters: a=7.417(3) angstrom, b=7.668(2) angstrom, c=7.744(5) angstrom, alpha=71.59(3)degrees, beta=87.71(4)degrees and gamma=86.04(6)degrees. This structure is characterized by HSeO4- and disordered (HxSe/P)O-4- tetrahedra connected to dimers via hydrogen bridges. These dimers are linked and stabilized by additional hydrogen bonds (O-H-O) and hydrogen bridges (O-H...O) to build chains of dimers which are parallel to the [0, 1, 0] direction at the position x=0.5. The differential scanning calorimetry diagram showed two anomalies at 493 and 563 K. These transitions were also characterized by optical birefringence, impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of the proton conductors in this compound were determined in a wide temperature range. The transport properties in this material are assumed to be due to H+ protons hopping mechanism. (c) 2005 Elsevier B.V. All rights reserved.