A differential calorimeter has been constructed to measure the heats of dilution of electrolytic solutions. The differential method made possible the determination of heats of dilution to within a few thousands of a calorie. Precision of this order was necessary to obtain meaningful values for the thermodynamic properties of very dilute solutions;The heats of dilution of neodymium chloride and erbium chloride solutions have been measured at 25°C for concentrations up to 0.2 molal. Empirical expressions for the relative apparent molal heat content of the solute, &phis; L; the relative partial molal heat content of the solvent, L¯1; and the relative partial molal heat content of the solute, L¯2, have been derived from the heats of dilution. The following equations have been derived for neodymium chloride solutions: fL=6925 m1/2-16725 m+22278m3/2 -7596m2, 74 L1&d1;= -62.37m 3/2+301.32m2 -602.04m5/2+273.70 m3 ,80 L2&d1;=10387 m1/2-33450 m+55695m 3/2+22788 m2. 79 The corresponding equations for erbium chloride solutions were as follows: fL=5687 m1/2-8637m +7066m3/2, 87 L1&d1;=-51.2 3m3/2+155.60 m2-190.95 m5/2, 88 L2&d1;=8531 m1/2-17274 m+17665m 3/2. 89;The thermodynamic properties of neodymium chloride and erbium chloride solutions were the same, within experimental error, for concentrations above 0.02 molal. For concentrations below 4 x 10-4 molal, the thermodynamic properties of neodymium chloride solutions followed the Debye-Huckel law predictions, but the thermodynamic properties of erbium chloride solutions appeared to depart from theoretical predictions.