Relatively little information is available in the literature regarding the speciation and solubility of tantalum in aqueous solutions. The available thermodynamic data have been critically evaluated and entered into a thermodynamic database. The JESS suite of computer programs has been used to achieve thermodynamic consistency and provide a critically-selected set of equilibrium constants that can later be used for modelling purposes. The utility of the resulting set is limited by a lack of adequate data. Gathered evidence suggests however that tantalum concentrations in freshwater are probably controlled by the changing solubility of tantalum oxide solid as it ages.