The apparent molar heat capacities and apparent molar volumes of Nd(ClO4)(3)(aq), Eu(ClO4)(3)(aq), Er(ClO4)(3)(aq), and Yb(ClO4)(3)(aq) have been measured at the temperatures T = (283.2, 298.2, 313.2, and 328.2) K and molalities from approximate to 0.05 mol.kg(-1) to approximate to 0.7 mol.kg(-1) in a Picker flow microcalorimeter and vibrating tube densimeter. The results were analysed by means of the Pitzer equations to derive standard partial molar heat capacities C(p,2)degrees, standard partial molar Volumes V-2 degrees and expressions for the excess properties. The revised Helgeson-Kirkham-Flowers (HFK) model has been used to represent the temperature dependence of the standard partial molar properties within the experimental uncertainty. Plots of C(p,2)degrees and V-2 degrees at T = 298.15 K against the ionic radii of La3+, Nd3+, Eu3+, Er3+, Gd3+, and Yb3+ clearly display the discontinuous behaviour known as the "gadolinum break". The ionic-radius dependence of the standard partial molar volumes is consistent with changes in the primary hydration number. The effect of temperature on the behaviour of the standard partial molar heat capacities across the series suggests that secondary sphere hydration has a major effect on the values observed for C(p,2)degrees.