The solubility of meso-1,2,3,4-butanetetracarboxylic acid (BTCA) in water and the nature of the solid phases in BTCA-water mixtures were studied. The form of BTCA that was isolated after crystallizing from a saturated aqueous solution and remaining in contact with the solution for over 4 years was BTCA(.)H(2)O. At 25 degreesC, a saturated aqueous solution of this product contained 6.99% BTCA. Other hydrates of BTCA were isolated after equilibration of BTCA with water at 25 degreesC for 1 h to 36 days. Thermal analysis identified these products as BTCA(.)2H(2)O after 1 h, BTCA(.)4H(2)O or BTCA-2H(2)O after 5 h, and BTCA(.)H(2)O after 15 and 36 days. The BTCA concentration in the liquid phase decreased from 18.67% after 5 h to 10.32% after 36 days. A saturated solution of BTCA(.)2H(2)O contained 18.38% BTCA. Several thermograms for BTCA(.)H(2)O or BTCA(.)2H(2)O contained endothermic peaks that are attributed to transitions between forms of anhydrous BTCA.