Following the procedure given in the literature, we prepared water-soluble Th symmetric derivative of fullerene C-60, at which six malonic acid molecules have been attached, T-h-C-60(C(COOH)(2))(6). The sample has been characterized by some fundamental physicochemical data and properties : C-13 NMR chemical shifts for the acid and its sodium salt, solubility data, potentiometric and conductometric titration curves, the electrical conductivity, the osmotic coefficient, and the apparent molar volume. It has been found that fullerenehexamalonic acid (FHMA), C-66(COOH)(12), behaves as a diprotic acid. The first carboxylic group, COOH(1), of the malonic acid functional,group behaves as an almost strong acid, whereas the second COOH(2) group is a weak acid with pK(2) of about 5.5. The molar conductivity extrapolated to zero concentration yielded Lambda(0) = 395.0 Omega(-1) cm(2) mol(-1) COOH(1) and the molar conductivity of half-ionized fullerenehexamalonate ion Lambda(0) (1/6C(66)(COOH)(6)(COO-)(6)) = 45.2 Omega(-1) cm(2) mol(-1). Due to the low solubility of the acid (about 11.8 g/L) the measurements were restricted to a narrow concentration range from about 0.0018 to 0.00001 mol FHMA/L. In this concentration range, the apparent molar volume was found to be constant and equal to phi nu = 557 mL/mol FHMA. The osmotic coefficient, which we succeeded in determining only at the highest concentration, is comparable to the fraction of free hydrogen ions calculated from conductivity data and pH measurements.