Aqueous calcium uranyl carbonate complexes are the predominant form of U(VI) in neutral to alkaline water in the environment, including seawater. We report a robust and repeatable synthesis of a nanoscale [(UO2)24O4(OH)12(CO3)36]44− cage cluster that we co-crystallized with Ca and Na countercations and determine its structure using single-crystal X-ray diffraction. Crystals were harvested and dissolved in water, followed by collection of electrospray ionization-mass spectra (ESI-MS). Introduction of a pH 8.1 solution containing 1.19 ± 0.04 mmol·L−1 U into the spectrometer produced broad m/z peak envelopes with −7 and 6 charge states at 1375, 1430, 1530, 1600, and 1675 m/z corresponding to mass 9625, 10,010, 10,710, 9600 and 10,080 amu, respectively. The spectra are consistent with [(UO2)24O4(OH)12(CO3)36]44− in solution (8909.07 amu), together with charge-balancing cations and H2O. Intentional fragmentation of the clusters yielded uranyl hydroxide and uranyl carbonate species containing Na. The [(UO2)24O4(OH)12(CO3)36]44− cluster is similar to that found in the mineral ewingite, which has been found to form in mine waters of an abandoned uranium mine. Demonstration of persistence of a nanoscale Ca uranyl carbonate cage cluster in aqueous solution reveals the need to re-examine aspects of environmental metal speciation to evaluate the extent to which nanoscale clusters may be important. SynopsisThis study demonstrates the existence of a nanoscale aqueous uranyl carbonate macroion at environmentally relevant pH conditions using mass spectrometry.