The heats of dilution of aqueous LaCl/sub 3/, PrCl/sub 3/, NdCl/sub 3/, SmCl/sub 3/, EuCl/sub 3/, GdCl/sub 3/, TbCl/sub 3/, DyCl/sub 3/, HoCl/sub 3/, ErCl/sub 3/, TmCl/sub 3/, YbCl/sub 3/, and LuCl/sub 3/ solutions have been measured up to saturation at 25/sup 0/C. The integral heats of solution of LaCl/sub 3/ . 7H/sub 2/O, PrCl/sub 3/ . 7H/sub 2/O, NdCl/sub 3/ . 6H/sub 2/O, SmCl/sub 3/ . 6H/sub 2/O, EuCl/sub 3/ . 6H/sub 2/O, GdCl/sub 3/ . 6H/sub 2/O, TbCl/sub 3/ . 6H/sub 2/O, DyCl/sub 3/ . 6H/sub 2/O, HoCl/sub 3/ . 6H/sub 2/O, ErCl/sub 3/ . 6H/sub 2/O, TmCl/sub 3/ . 6H/sub 2/O, YbCl/sub 3/ . 6H/sub 2/O, and LuCl/sub 3/ . 6H/sub 2/O in water at 25/sup 0/C have also been measured. The heat of dilution data are represented by empirical equations, and relative partial molal heat contents are calculated. The heat content trends across the rare earth chloride series are similar to the trends found for the rare earth perchlorate heat data, and can be correlated with a change in the inner sphere cation water coordination across the rare earth cation series. 11 figures, 3 tables, 43 references.