The solubility of uranyl minerals controls the transport and distribution of uranium in many oxidizing environments. Uranyl minerals form as secondary phases within uranium deposits, and they also represent important sinks for uranium and other radionuclides in nuclear waste repository settings and at sites of uranium groundwater contamination. Standard state Gibbs free energies of formation can be used to describe the solubility of uranyl minerals; therefore, models of the distribution and mobility of uranium in the environment require accurate determination of the Gibbs free energies of formation for a wide range of relevant uranyl minerals. Despite decades of study, the thermodynamic properties for many environmentally-important uranyl minerals are still not well constrained. In this review, we describe the necessary elements for rigorous solubility experiments that can be used to define Gibbs free energies of formation; we summarize published solubility data, point out difficulties in conducting uranyl mineral solubility experiments, and identify areas of future research necessary to construct an internally-consistent thermodynamic database for uranyl minerals. (c) 2007 Elsevier Ltd. All rights reserved.