A thermodynamic analysis of CsHSO4 in oxygen and hydrogen atmospheres has been carried out. In an oxygen atmosphere, the phase relationship for the dehydration of CsHSO4 as a function of H2O partial pressure was established. It is concluded that CsHSO4 is stable in a humidified oxygen atmosphere, pH(2)O = 0.03 atm, up to 170 degreesC. However, in hydrogen atmospheres, it was affirmed that CsHSO4 is reduced to H2S and Cs2SO4. Electromotive force (EMF) measurements in a humidified oxygen concentration cell, pH(2)O = 0.03 atm confirmed the ionic nature of the charge-carrying species. Upon thermal cycling, the voltage obtained from the high temperature phase (T = 149 degreesC) remained unchanged for over 85 h of measurement. The volume change upon transformation to the superprotonic phase, as directly measured by high temperature X-ray powder diffraction, is +0.54%. These results demonstrate not only the stability of CsHSO4 in oxygen but also the viability of solid acid-based electrochemical devices that would likely be subjected to thermal cycling during operation. (C) 2004 Elsevier B.V. All rights reserved.