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Uranyl minerals form as a result of the oxidation and alteration of uraninite, UO 2+x . These uranyl phases are also important alteration products of the corrosion of UO 2  in used nuclear fuels under oxidizing conditions. However, the thermodynamic database for these phases is extremely limited. The Gibbs free energies and enthalpies for uranyl phases are estimated based on a method that sums polyhedral contributions. The molar contributions of the structural components to ∆G 0  f  and ∆H 0  f  are derived by multiple regression using the thermodynamic data of phases for which the crystal structures are known. In comparison with experimentally determined values, the average residuals associated with the predicted ∆G 0  f  and ∆H 0  f  for the uranyl phases used in the model are 0.08 and 0.10%, respectively, well below the limits of uncertainty for the experimentally determined values. To analyze the reliability of the predicted ∆G 0  f  values, activity-activity diagrams in SiO 2 -CaO-UO 3 -H 2 O and CO 2 -CaO-UO 3 -H 2 O systems at 298.15 K and 1 bar were constructed using the predicted ∆G 0  f,298.15  values for the relevant U 6+  phases. There is good agreement between the predicted mineral stability relations and field occurrences, thus providing confidence in this method for the estimation of ∆G 0  f  and ∆H 0  f  of the U 6+  phases.

The Gibbs free energies and enthalpies of formation of U 6+ phases: An empirical method of prediction