The solubility of uranium-containing minerals can control the mobility of uranium in contaminated soil and groundwater. The identity and solubility of these minerals are strongly influenced by solution composition. The influence of dissolved sodium and cesium on the solubility of uranyl oxide hydrates has been investigated in a series of batch experiments conducted with synthetic metaschoepite ((UO2)8O2(OH) 12·10H2O). During reaction of metaschoepite in NaNO3, CsNO3, and NaF solutions, an initial increase in the dissolved uranium concentration was followed by a decrease as uranium was incorporated into a secondary solid phase. Given sufficient reaction time, metaschoepite was completely transformed to a clarkeite-like sodium uranyl oxide hydrate or a cesium uranyl oxide hydrate that has not previously been described. These secondary solid phases exhibited X-ray diffraction patterns and Raman spectra that were distinct from those of the original metaschoepite. Dissolved uranium concentrations in equilibrium with the sodium and cesium uranyl oxide hydrates can be more than 2 orders of magnitude lower than those in equilibrium with metaschoepite. Initial changes in metaschoepite solubility may also result from particle growth induced by sodium and cesium incorporation into the solid phase.