The standard heat of formation of aqueous orthosphosphoric acid H3PO4 . 0·6416 H2O has been determined by solution calorimetry using a reaction scheme involving hexagonal P4O10 and water. The value obtained ΔH°f298 H3PO4.0.6416 H2O =–305.9±0.5 kcal mole–1 has a large uncertainty due to the accuracy assigned to the standard heat of formation of hexagonal P4O10.