The available equilibrium constant data for reactions of niobium in aqueous solution have been critically evaluated and entered into a thermodynamic database. The relevant literature is sparse and, where comparable, often inconsistent. Problems include poor characterisation of solid phases and limitations with older analytical techniques. Nevertheless, the JESS suite of computer programs has been applied where possible to achieve thermodynamic consistency and provide a critically-selected set of equilibrium constants that could be useful for modelling purposes. However, the utility of the resulting set is evidently limited by a lack of adequate data. This is unfortunate given the significance of niobium in radioactive waste management and certain catalytic applications.