The crystal structures have been determined for the isomorphous pair, calcium sodium galacturonate (I) and strontium sodium galacturonate (II), as models for the biologically important interactions between Ca²⁺ and the pyranose forms of sugar carboxylic acids. The space group is P6₃ and the cell dimensions are: a= 13·56(1), c= 9·70(1)Å(I): and a= 13·50(1), c= 9·63(1)Å(II). There are six galacturonate anoions, twelve water molecules, two sodium ions, and two alkaline-earth cations in the hexagonal unit cell. The structures were solved by Fourier methods from photographic data, and refined by least-squares techniques to R 12·2 [(I), 837 observed reflections] and 8·4%[(II), 946 observed reflections]. Each divalent cation is co-ordinated by three sugar anions and three water molecules in a nine-co-ordinate trigonal prism arrangement. The sugar oxygen atoms which co-ordinate to the cation are those of the carboxylate and the sugar ring. We suggest that sugar anions will in general co-ordinate to Ca²⁺ through the carboxylate oxygen and the oxygen function on the α-carbon atom. With these two oxygens in an eclipsed or near-eclipsed conformation. In solution, however, the system may deviate from the eclipsed conformation to facilitate a terdentate complex with its entropic advantage.