The standard free energy of formation of pyrophyllite, ∆G°f(Pr), is calculated from reversed hydrothermal data. The value falls into two distinct groups: about —1251 kcal from breakdown of kaolinite+quartz, and about —1258 kcal from pyrophyllite breakdown into andalusite+quartz, or pyrophvllite+corundum into kyanite. The second value agrees with a direct determination of ∆Gf°(Pr) by solubility measurements. The discrepancy cannot be fully explained by uncertainties in the calculations, in the location of experimental points, or by uncertainties in the entropy and volume data used in the calculations. Instead, large systematic errors appear to exist in the values of free energy of formation for kaolinite and/ or the polymorphs of Al2SiO5. Petrographic data, namely the apparent stability of kaolinite +(|uartz relative to pyrophyllite in many rocks, and the common occurrence of muscovite +quartz and rarity of microcline+pyrophyllite, suggest that the error is in the data for the Al2SiO5 polymorphs.