There are very few examples in nature for U(VI) compounds with carbonate ligands other than the well known tricarbonates. Especially examples of U(VI) dicarbonato compounds are nearly completely missing. Even in aqueous solutions, the dicarbonato complex was found as a species of minorimportance only. On the basis of structural data on the ligands H2O and carbonate as well as the available data on U(VI) coordination compounds, steric requirements of equatorial coordination are studied for aqueous solution species. A pentagonally coordinated monocarbonato species [UO2CO3(H2O)3] is found as the most likely coordination. For the dicarbonato species, hexagonally coordinated [UO2(CO3)2(H2O)2] with D2h symmetry is found as most probable structure. Possible causes of the instability of U(VI) dicarbonato species are discussed.