The Cs2HPO4 · 2H2O single crystals synthesized from an aqueous solution containing equimolar amounts of H3PO4 and Cs2CO3 were studied by impedance and IR spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry (DSC). The IR spectra were analyzed in accordance with the structural data, and the absorption bands were assigned. The proton conductivity was studied at temperatures in the range 20–250°C. The conductivity of dehydrated Cs2HPO4 was low, ~10–5–10–9 S cm–1 at 90–250°C with an activation energy of conductivity E a = 1.1 eV at 130–250°C. The processes determining the character of the temperature dependence of conductivity were consistent with the DSC and thermogravimetry data. According to these data, dehydration of the crystalline hydrate Cs2HPO4 · 2H2O starts at 60°C and occurs in three stages, forming Cs2HPO4 · 1.5H2O below 100°C; anhydrous Cs2HPO4 at t > 160°C, which is stable up to 300°C; and Cs4P2O7 above 330°C.