Using high‐temperature solution calorimetry in molten 2PbO. B2O3, the enthalpy of reaction of the formation of zircon, ZrSiO4, from its constituent oxides has been determined: ΔrH977(ZrSiO4) =−27.9 (± 1.9) kJ/mol. With previously reported data for the heat contents of ZrO2, SiO2, and ZrSiO4and standard‐state enthalpies of formation of ZrO2and SiO2, we obtain ΔfH°298· (ZrSiO4) =−2034.2 (±3.1) kJ/mol and ΔfG°298(ZrSiO4) =−1919.8 kJ/ mol. The free energy value is in excellent agreement with a range previously estimated from solid‐state reaction equilibria. At higher temperature also the data are in close agreement with existing data, though the data sets diverge somewhat with increasingT. The limitations of the data for predicting the breakdown temperature of zircon into its constituent o