The standard Gibbs energy of formation of zircon was constrained by measuring the solubility of silica in H 2 O in equilibrium with zircon and baddeleyite at 800 °C, 12 kbar, by a sensitive weightchange method. Dissolution occurs incongruently according to the reaction: ZrSiO 4 = ZrO 2 + SiO 2(t) zircon baddeleyite where SiO 2(t) is total dissolved silica. Blank runs demonstrated that the effects of ZrO 2 solubility and/or capsule-Pt transfer were near the weighing detection limit, so weight losses or gains could be ascribed quantitatively to SiO 2 solubility. Precise SiO 2(t) concentrations were ensured by use of three types of starting material, by approaching equilibrium from zircon-undersaturation and oversaturation, and by demonstrating time-independence of the measurements. The results yielded a SiO 2 concentration of 0.069 ± 0.002 (1 se) moles per kg H 2 O (m s ), or a mole fraction (X s ) of 1.23 × 10 -3 ± 3.3 × 10 -5 . Two runs on zircon solubility in NaCl-H 2 O solutions at 800 °C and 10 kbar showed silica solubility to decrease by nearly 1% per mol% NaCl. The standard molar Gibbs free energy of formation of zircon from the oxides at a constant P and T is given by: where ZB and Q refer, respectively, to equilibrium with zircon-baddeleyite and quartz, γ s is the activity coefficient of total silica, and the relationship between γ s and X s accounts for aqueous silica activity. Our results yield ΔG ○ f,ox,zr = -18.5 ± 0.7 kJ/mol at 800 °C, 12 kbar (95% confidence), or a standard apparent Gibbs free energy of formation from the elements of -1918.3 ± 0.7 kJ/mol at 25 °C, 1 bar. Our value is consistent with determinations based on phase equilibrium studies, within reported error limits, but is more precise than most previous values. However, it is less negative than high-temperature determinations by calorimetry and electrochemistry. Our results indicate that solubility measurements at high T and P may be a superior method of free energy determination of other refractory silicate minerals.