Abstract In agreement with previous work on aluminum and boron phosphates, Ca(H 2PO 4) 2·H 2O undergoes dehydration and/or hydrolysis in the solid state under water vapour pressure at 200–600°C. At 5 atm. steam pressure calcium dihydrogen phosphate and Ca 2HP 3O 10 exist up to 300 and 400°C respectively. At 500°C no acid phosphates have been detected. Neutral phosphates such as Ca 2P 2O 7 are suggested as possible stabilizers of the protonic salts. The results are relevant in relation to the synthesis of high temperature stable proton conducting solid electrolytes.