Spent UO2 fuel will rapidly be altered to U6+ phases in nuclear waste repositories. Be-cause most uranyl phases are based on sheet or chain structures and usually contain several mo-lecular water groups, site-mixing, vacancies, as well as disorder in the orientation of hydrogen bonds may occur. A systematic survey of the published crystallographic data for uranates, uranyl oxide hydrates, phosphates, silicates, carbonates, and sulfates demonstrates that site-mixing ap-parently occurs in the structures of at least 31 uranyl phases. Calculations of the ideal site-mixing entropy indicate that the residual contribution that arises from substitution and vacancies to the third-law entropies of some uranyl phases is large. A brief examination of the crystal chemistry of water molecules in uranyl phases suggests that considerable residual entropy may be caused by the disorder of hydrogen bonds associated with interstitial H2O groups. In the geochemical envi-ronment that expected to occur in the near-field of nuclear waste repositories, the existence of structure-configurational entropy may reduce the uranium concentration of several log units in so-lutions equilibrated with some uranyl phases. Therefore, compositional analysis and structural de-terminations must be made on the samples used in calorimetric measurements, and the calorimet-ric data must be combined with solubility data to evaluate the thermodynamic stability of the inter-ested phases. Spent UO2 fuel will rapidly be altered to U6 + phases in nuclear waste repositories. Be-cause most uranyl phases are based on sheet or chain structures and usually contain several mo-lecular water groups, site-mixing, vacancies, as well as disorder in the A systematic survey of the published crystallographic data for uranates, uranyl oxide hydrates, phosphates, silicates, carbonates, and sulfates demonstrates that site-mixing ap-parently occurs in the structures of at least 31 uranyl phases. Calculations of the ideal site-mixing entropy indicate that the residual contribution that arises from substitution and vacancies to the third-law entropies of some uranyl phases is large. A brief examination of the crystal chemistry of water molecules in uranyl phases suggests that considerable residual entropy may be caused by the disorder of hydrogen bonds associated with interstitial H2O groups. In the geochemical envi-ronment that expected to occur in th The near-field of nuclear waste repositories, the existence of structure-configurational entropy may reduce the uranium concentration of several log units in so-lutions equilibrated with some uranyl phases. Thus, compositional analysis and structural de-terminations must be made on the samples used in calorimetric measurements, and the calorimet-ric data must be combined with solubility data to evaluate the thermodynamic stability of the inter-ested phases.