Following the procedure given in the previous paper of this series, we prepared the sodium salt of fullerenehexamalonic acid, C-66(COONa)(12) For aqueous solutions of this electrolyte the electrical conductivity, the osmotic coefficient, the apparent molar volume, and the enthalpy of dilution have been determined at 25 degrees C in the concentration range from about 0.3 to 0.001 mol of COO-/L. It has been found that C-66(COONa)(12) behaves as a strong highly asymmetric electrolyte. The concentration dependence of the apparent molar volume and the enthalpy of dilution displays minima at concentrations about 0.004 and 0.005 mol of COO-/L, respectively. The appearance of the minima has been attributed to the partial hydrolysis of C-66(COONa)(12). The experimental data have been compared with the theoretical predictions based on the solution of the Poisson-Boltzmann equation for the spherical cell model. A reasonable agreement between experiment and theory has been observed.