The high temperature behavior of the pseudo-binary system Rb3H(SO4)(2)-RbHSO4 has been investigated through a combination of thermal analysis, high-temperature X-ray powder diffraction and high-temperature electrical impedance measurements. The studies reveal the existence of a new high-conductivity phase, Rb5H3(SO4)(4,) that becomes stable at a temperature between 25 and 140 degrees C and is isostructural to a recently reported trigonal phase of Cs5H3(SO4)(4) [M. Sakashita, et al., Solid State Ionics (2007) 178 1262-1267]. Refinement of the Rb structure in space group P (3) over barm at 160 degrees C yields a = 5.9298(2) angstrom, c = 14.4736(6) angstrom, V = 440.74(3) angstrom(3), with Z = 1 and rho = 3.0691(2) Mg/m(3), At 185 degrees C the compound undergoes an apparently polymorphic transformation with a heat of transition of 14.9 kJ/mol and a slight additional increase in conductivity. No further anomalies are observed up to the highest measurement temperature of 220 degrees C. Compositions rich in Rb3H(SO4)(2) relative to Rb5H3(SO4)(4) show, in addition to the polymorphic transformation of the latter, the disproportionation reaction of the former into Rb2SO4 and Rb5H3(SO4)(4) at 205 degrees C. Compositions rich in RbHSO4 display an additional conductivity anomaly at 170 degrees C, which is tentatively assigned to a polymorphic transformation in RbHSO4. (c) 2011 Elsevier B.V. All rights reserved.