The MH(AO) acid salts (M = Cs, Rb, K, Na, Li, NH; A = S, Se, As, P) exhibit ferroelectric properties. The solid acids have low conductivity values and are of interest with regard to their thermal properties and proton conductivity. The crystal structure of caesium dihydrogen orthophosphate monohydrogen orthophosphate dihydrate, Cs(HPO)·2HO, has been solved. The compound crystallizes in the space group Pbca and forms a structure with strong hydrogen bonds connecting phosphate tetrahedra that agrees well with the IR spectra. The dehydration of Cs(HPO)·2HO with the loss of two water molecules occurs at 348-433 K. Anhydrous Cs(HPO) is stable up to 548 K and is then converted completely into caesium pyrophosphate (CsPO) and CsPO. Anhydrous Cs(HPO) crystallizes in the monoclinic C2 space group, with the unit-cell parameters a = 11.1693 (4), b = 6.4682 (2), c = 7.7442 (3) Å and β = 71.822 (2)°. The conductivities of both compounds have been measured. In contrast to crystal hydrate Cs(HPO)·2HO, the dehydrated form has rather low conductivity values of ∼6 × 10-10 S cm at 373-493 K, with an activation energy of 0.91 eV.