The standard formation enthalpy of KH(PO3H) has been determined by solution calorimetry combining solution enthalpies of KH(PO3H) and KOH + H3PO3 mixtures in 2M HCl at 298.15K and literature data. The compounds above-mentioned are found to be thermo-dynamically stable as to their decomposition into binary oxides at room temperatures. It has been established that KH(PO3H) does not react with water and CO2 at ambient temperatures. (C) 2012 Published by Elsevier B.V.