The structural characteristics of the crystals (RbH2(PO4)(0.65)(AsO4)(0.35)) were analyzed by means of X-ray diffraction, which revealed that the new title compound is nearly isomorphous with the tetragonal phase of RbH(2)ASO(4) (RDA). The following results have been obtained: space group 1(-)42d, a = 7.6837 (8) angstrom, c = 7.3824 (19) angstrom, V = 435.85(13) angstrom(3) and Z = 4. The phosphorus and the arsenic atom as well as the rubidium ions lie on points with site symmetry S-4, the oxygen atoms lie in general positions about the phosphorus-arsenic atoms, in a tetrahedral arrangement. The (P/As)04 tetrahedra are connected by 0 H...0 hydrogen bonds laying essentially in the a,b plane. Differential scanning calorimetric traces show three phase transitions at 229, 390 and 580 K in this material. Moreover, protonic conduction of the mixed compound determined by impedance and modulus spectroscopy has been studied in the temperature range 150-450 K in order to determine the nature of the phase transition. (C) 2017 Elsevier B.V. All rights reserved.