The apparent molar heal capacities and volumes of LaCl3(aq), La(ClO4)(3)(aq), and Gd(ClO4)(3)(aq) have been measured at the temperatures (283, 298, 313, 328 and 338) K and molalities from about 0.05 mol . kg(-1) to 0.7 mol . kg(-1) in a Picker flow microcalorimeter and a vibrating tube densitometer. The results were analyzed by means of the Fitter equations to derive standard partial molar heat capacities and volumes and expressions for the excess properties. The revised Helgeson-Kirkham-Flowers (HKF) model has been found to represent the temperature dependence of the standard partial molar properties within the experimental uncertainty. A modification to the non-electrostatic heat capacity function in the HKF model yields an improved fit to the experimental results for La(ClO4)(3) and Gd(ClO4)(3).