The high temperature properties of Rb2SeO4 have been studied by calorimetry, impedance spectroscopy and X-ray powder diffraction. As synthesized, Rb2SeO4 includes a second phase of Rb2SeO4, which can be eliminated upon heating the compound. As expected, no conductivity is observed in dry (pH(2)O < 0.001 bar) air. By changing to humidified (pH(2)O = 0.1 bar) air at 176 degrees C, the conductivity increases sharply from 8.6.10' to 1.7-10(-6) S cm(-1). Under humidified (pH(2)O = 0.1 bar) reducing atmosphere (9%H-2 in N-2), the conductivity increases to 2.0.10(-4) S cm(-1) at 317 degrees C. Degradation of Rb2SeO4 and Rb2SeO4 to form RbOH, which is known as a proton conductor, are thought to be responsible for the observed conductivity in humidified atmospheres. Our observations may explain the conductivity rise in other solid acid electrolytes, including sulfate and selenate groups, around their decomposition temperatures, in humidified atmospheres. (C) 2012 Elsevier B.V. All rights reserved.