The dehydration behavior of the paraelectric phase of CsH2PO4 was investigated by thermogravimetric and X-ray diffraction analyses, and then the phase diagrams of CsH2PO4-CsPO3 system were established. The relationship between the onset temperature of dehydration (T-dehy/K) and the partial pressure of water (pH(2)O/atm) is logpH(2)O = 7.62 (+/- 1.18) -4.42 (+/- 0.56) (1000/T-dehy) below 228 degrees C. The thermodynamically stable phase just above T-dehy is the fully dehydrated product CsPO3(s), although partially dehydrated products transiently appeared in the course of the dehydration to CsPO3. Such developments allowed us to complete the temperature-humidity phase diagram and to regard the composition-temperature phase diagram as the eutectic type. This paper also reports the phase transition and electrical properties of CsPO3 examined by differential thermal analysis, ac impedance spectroscopy, and dc polarization measurement. The just-synthesized CsPO3 showed the relatively high electrical conductivity in unhumidified Ar. Ionic conductivity as high as 5 x 10(-4) S cm(-1) was observed on heating from 150 to 450 degrees C. Such high ionic conductivity disappeared after first heating up to 620 C and was explained by the proton diffusion through the absorbed H2O. At around 600 degrees C, a high-temperature phase of CsPO3 showed the electrical conductivity as high as 10(-3) S cm(-1). However, this conductivity was not purely ionic. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3086755] All rights reserved.