An adiabatically jacketed differential calorimeter, with a sensitivity of 5 x 10^-4 calories per millimeter pen displacement of a recording potentiometer, was used to measure the heats of dilution of solutions of lanthanum chloride, ytterbium chloride, lanthanum nitrate and ytterbium nitrate. The concentrations of the solutions ranged from about 0.10 molal to about 0.25 molal. The heats of dilution, which ranged from about 0.05 to 2.0 calories, were measured to within a few thousandths of a calorie.