Densities and volumetric heat capacities for aqueous solutions containing Th(ClO4)4 in dilute (similar to 0.12 m) HClO4 have been determined from 10 to 55 degrees C. The results have been analyzed using a simple molality fraction approach (Young’s rule) and an ion-interaction model (Pitzer ion-interaction scheme) that have led to calculated values for the apparent and partial molar volumes and heat capacities of Th(ClO4>(4)(aq) and subsequently Th4+(aq) over this same temperature range. These results show the most highly negative standard state (infinitely dilute) partial molar volumes and heat capacities for any monatomic aqueous ion. The results have been examined in reference to earlier semitheoretical correlations and X-ray diffraction results for the hydration of aqueous ions and are consistent with expected Coulombic and ion-solvent interactions.