An investigation of solid solutions of KH2AsO4-CsH2AsO4 has led to the discovery of a new compound Cs0.5K0.5H2AsO4 (denoted CKDA). The compound was characterized by X-ray single crystal analysis performed at room temperature and reveal that it crystallizes in the space group I- 42d with the following crystal data: a = 7.6288(6) angstrom, c = 7.1808 (7) angstrom, V = 417.91(6) angstrom(3) and Z = 4. The arsenic atom as well as the potassium and cesium ions lie on points with site symmetry S-4, the oxygen atoms lie in general positions about the arsenic atoms, in a tetrahedral arrangement. The AsO4 tetrahedra are connected by O-H center dot center dot center dot O hydrogen bonds laying essentially in the a,b plane. The infrared spectrum at room temperature confirms a structural disorders and the co-existence of the anions H2AsO43- in the crystal structure. A calorimetric study of the title compound shows two distinct endothermal peaks which are detected at 215 and 465 K. Moreover, protonic conduction of the mixed compound determined by impedance and modulus spectroscopy has been studied in the temperature range 180-473 K. (C) 2017 Elsevier B.V. All rights reserved.