The proton conduction and superionic phase transition of cesium dihydrogen phosphate, CsH2PO4 (CDP), were investigated under various humid conditions to evaluate the applicability of a CsH2PO4 solid electrolyte to an intermediate temperature fuel cell operating between 230 degrees C and 300 degrees C. The phase stability, superionic phase transition, and reversibility of dehydration of CsH2PO4 were evaluated under different ambient water vapor concentrations, from 0 to 90 mol%, through the measurements of conductivity. The dependence of conductivity on the water vapor concentration and the demonstrated reversibility of dehydration clearly showed the range in which CsH2PO4 is applicable to the intermediate temperature fuel cell. Additionally, we evaluated the protonic transport number of CsH2PO4, which was almost unity, and demonstrated fuel cell operation at 250 degrees C using a single cell fabricated with the CsH2PO4 electrolyte.