The poly-thermal solubility of the tris-malonate C60[=C(COOH)2]3 – H2O binary system was investigated from 20 – 80 ◦C with the help of the method of isotherm saturation in ampoules. Concentration of tris-malonate C60[=C(COOH)2]3 in solutions was determined by light absorption at 330 nm. A diagram of the solubility is non-monotonic, consisting of 2 branches, which correspond to 2 different crystal-hydrates of C60[=C(COOH)2]3 and one non-variant point, corresponding to the saturation both crystal-hydrates. Complex thermal analysis of C60[=C(COOH)2]3 crystal hydrates, in equilibrium with a saturated aqueous solution at room temperature, was performed from 20 – 600 ◦C. Consecutive effects of the losses of C=O and C=O + H2O were determined.