The influence of thermal cycle on the conduction behavior of CsH5(PO4)(2) was investigated. The temperature dependence of the conductivity was examined in two consecutive heating processes with different sequential operations under a dry atmosphere. In the second heating, the conduction behavior was significantly affected by the thermal history. This phenomenon could be explained consistently by the results from X-ray diffraction (XRD) and thermal analyses of CsH5(PO4)(2). When the sample was heated at the melting point of CsH5(PO4)(2) and then rapidly cooled, the diffraction lines of CsH5(PO4)(2) and the condensed phosphates were observed in the XRD pattern. The differential scanning calorimetry and thermogravimetry-differential thermal analysis were conducted in the heating-cooling process under various experimental conditions. It was noted that the crystallization temperature of CsH5(PO4)(2) was strongly dependent on the extent of dehydration. (C) 2008 The Electrochemical Society.