The high temperature properties of Rb3H(SO4)(2) have been studied by calorimetry, impedance spectroscopy and X-ray powder diffraction under moderate humidification. At similar to 205 degrees C the conductivity of Rb3H(SO4)(2) increases sharply, rising from 3.3 x 10(-5) to 1.9 x 10(-3) S/cm, suggestive of a polymorphic, superprotonic phase transition. This conductivity anomaly is accompanied by an endothermic thermal event with a heat of transition of similar to 18 kJ/mol. The X-ray powder diffraction pattern of Rb3H(SO4)(2) collected at 214 degrees C, however, shows peaks that can be attributed to Rb2SO4 and an unknown solid phase. The results indicate that, rather than a polymorphic transition, the conductivity increase of Rb3H(SO4)2 corresponds to solid state disproportionation, described as Rb3H(SO4)(2)(S)-> Rb2SO4(S)-> RbmHn(SO4)(p)(S), where the phase of unknown composition is rich in sulfuric acid relative to Rb3H(SO42). Drop solution calorimetry, carried out using molten sodium molybdate as the solvent, revealed the enthalpy of the alternative reaction Rb3H(SO4)(2)(S)-> Rb2SO4(S)+RbHSO4(s) to be essentially zero (0.9 +/-2.7 kJ/mol), supporting the assertion that the observed transformation involves different product phases. The standard enthalpy of formation of Rb3H(SO4)(2) from the elements at 25 degrees C was found to be similar to 2602 +/- 10 kJ/mol. (C) 2008 Elsevier B.V. All rights reserved.