When chlorine (real gas, 980 mmHg) combines with white (α) phosphorus to form phosphorus pentachloride at 24.89 °C, 3986.3 ± 2.7 J of heat are liberated per gram of stoichiometrically equivalent Mg2P2O7. This value was determined by a dead-ended flow system in an electrically calibrated isoperibol calorimeter. Correction was made for the formation of up 10 one mol percent PCl3 in the products; no other impurities were found. From this value, the standard (25 °C, ideal gas) enthalpy of formation of PCl5(c) is calculated to be -443.85 ± 0.30 kJ/mol, -106.08 ± 0.07 kcal/mol. When this figure is combined with a recent determination of the enthalpy of hydrolysis of PCl5(c), the standard enthalpy of formation of H3PO4 (aq, 100 H2O) is calculated to be -1296.5 ± 1.5 kJ/mol, in good agreement with two other values which involve the formation and hydrolysis of P4O10 (hex). A "best value" is suggested: ΔH f° (H3PO4, 100 H2O) = -1295.8 ± 1.3 kJ/mol, -309.7 ± 0.3 kcal/mol.