Abstract Investigation of solid solutions of CsHSeO 4–CsH 2PO 4 have led to the discovery of the compound Cs 2(HSeO 4)(H 2PO 4) (denoted CsHSeP). The structural properties of the obtained crystals were characterized by single-crystal X-ray analysis: Cs 2(HSeO 4)(H 2PO 4) crystallizes at room temperature in the space group P2 1/ c with lattice parameters a = 12.256(7) Å, b = 7.883(6) Å, c = 10.140(6) Å, β = 90.32(5)°, Z = 4 and V = 979.6(1) Å 3. In this structure, the SeO 4 and PO 4 tetrahedra are connected by O–H⋯O hydrogen bonds, to a zigzag chains running in the b-direction. These chains are also linked by hydrogen bridges to form layers parallel (1 0 0). The thermal-differential analysis of the superprotonic transition in Cs 2(HSeO 4)(H 2PO 4) showed that the transformation to high-temperature phase occurs at 400 K by one-step process. Thermal decomposition of the product takes place at 471 K. This decomposition occurs in several stages and characterized by weight loss of (CsHSeP). The first two transitions were also studied by X-ray powder diffraction at various temperatures and by impedance and modulus spectroscopy techniques. ac-impedance measurements revealed that, upon heating, the compound undergoes a transformation into a phase of high conductivity at ∼400 K. The activation energy increases from 0.12 to 0.24 eV, while the conductivity jumps from 2.60 × 10 −5 Ω −1 cm −1 at 393 K to 2.77 × 10 −4 Ω −1 cm −1 at 403 K. Information about charge carrier transport mechanism is obtained by comparison of Δ E f with Δ E σ . The activation energies for the (CsHSeP) compound calculated respectively from the modulus and impedance spectra are approximately close, suggesting that transport properties above and below the superprotonic phase transition (400 K) is probably due to H + protons hopping mechanism.