Rb3D(SeO4)(2) belongs to the well-known group of hydrogen sulfate and selenate crystals which are promising for fuel cell applications. However, the high temperature properties of this salt have been much less extensively studied than those of Rb3H(SeO4)(2), Superionic phase transition in Rb3D(SeO4)2 and Rb3H(SeO4)(2) single crystals was studied using impedance spectroscopy and DSC methods. Temperature evolution of the ferroelastic domain structure was observed under a polarizing microscope. Additionally, the X-ray diffraction and Raman scattering measurements for both crystals were carried out at room temperature. Rb3D(SeO4)(2) undergoes a structural phase transition from the low temperature ferroelastic phase to the superionic, paraelastic one. A correlation was found between the ferroelastic domains evolution, the anomalies of DSC and the temperature dependence of ionic conductivity. The temperature range of superionic phase transition in the first heating run was much wider and the Ts value was lower in Rb3D(SeO4)(2) than in the non-deuterated compound. Additionally, a considerable temperature hysteresis of Ts in Rb3D(SeO4)(2) was observed, unlike in Rb3H(SeO4)(2). The activation energies of H+ and D+ conductivity along the c-axis were found to be almost the same. The temperature variation of ionic conductivity in Rb3D(SeO4)(2) obtained in the first heating run of a virgin sample was found to differ considerably from that measured in subsequent cooling/heating runs. The small H/D isotope effect of proton conductivity in the crystals studied pointed to a classical activated diffusion mechanism. (c) 2007 Elsevier B.V. All rights reserved.