The dehydration of the superprotonic conductor CsH2PO4 has been carefully examined above 260 degrees C under highly humidified conditions. A.C. impedance spectroscopy on heating revealed that the dehydration of CsH2PO4 was accompanied by an increase in conductivity, whereas at temperatures below 260 degrees C, CsH2PO4 dehydrates to solid CSPO3 with decrease of conductivity. Based on a combination of thermogravimetric analyses, it was clarified that this phenomena was caused by the dehydration of CsH2PO4 to form a liquid phase as follows: CsH2PO4(s) -> CsH2(1-x) PO4-x(l) +xH(2)O(g). It was also determined that the liquid phase dehydrates to solid CSPO3 at higher temperatures and the stable region of CsH2PO4, dehydration liquid, and CsPO3 were established in the diagram of temperature and partial pressure of water. In addition, the electrical property of the dehydration liquid was measured by a.c. impedance spectroscopy as well as the d.c. polarization method. The liquid phase exhibited high ionic conductivity, 5(+/- 4) x 10(-2) S cm(-1), at 283.5 degrees C and p(H2O) = 0.3 atm. (c) 2007 Elsevier B.V. All rights reserved.