Calorimetric measurements have been made of the integral heats of solution at 25°C of the ethylsulphates of all the stable lanthanide elements, and of the corresponding bromates (except those of cerium and thulium). All of these salts are enneahydrates, but some are prone to crystallize with occluded mother liquor. To make it possible to correct for this, measurements have also been made of the solubilities of some bromates and ethylsulphates at 25°C and of the heat of dilution of some saturated solutions. The results have been used to estimate the heats of formation of lanthanide complexes in dilute solution starting with the rare-earth as the solid ethylsulphate or bromate. Where there are grounds for thinking that the complexes in solution are partially hydrated, the dependence of these heats of formation on the atomic number of the lanthanide is not a simple one. For the 1 : 3 diglycollate and 1 : 3 dipicolinate complexes, however, where the ligands probably occupy all the co-ordination positions, these heats of formation show a general decrease with increasing atomic number. Moreover, it appears that relative to the complexes of lanthanum, gadolinium and lutenium, these 1 : 3 complexes of the other rare-earth metals have heats of formation which are lower by amounts of the order of a few hundred calories, and it is suggested that this is due to the interaction of the crystal field of the ligands with the 4 f electrons of the lanthanide.