Commercial disodium hydrogen phosphate dodecahydrate (Na2HPO4 center dot 12H(2)O) was used as a precursor for synthesizing disodium hydrogen phosphate hexahydrate (Na2HPO4 center dot 6H(2)O) and sodium diphosphate (Na4P2O7). The purity of the synthesized products was checked up by IR spectroscopy and X-ray diffraction. The heat of dissolution of these compounds, in acid solutions of several concentrations (w/w) of H3PO4 was measured in a C-80 SETARAM calorimeter. Many dilution and mixing processes were also realized in the calorimeter in order to get the standard enthalpy of formation of these products. The values obtained for the enthalpies of formation are: (-3210.5) and (-3516.5) kJ . mol(-1) for sodium diphosphate (Na4P2O7) and disodium hydrogen phosphate hexahydrate (Na2HPO4 center dot 6H(2)O), respectively. (C) 2010 Elsevier Ltd. All rights reserved.