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, pH₂O=0.03 atm, up to 170 °C. 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, pH₂O=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 °C) 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.