Fourteen rare earth complexes with pyromellitic acid were synthesized and characterized by means of chemical and elemental analysis, and TG-DTG. The constant-volume combustion energies of complexes, Delta U-c. were measured by a precise rotating-bomb calorimeter (RBC-type II). Their standard molar enthalpies of combustion. Delta H-c(m)theta, and standard molar enthalpies of formation, Delta H-f(m)theta, were calculated at T = 298.15 K. The relationship of Delta H-c(m)theta and Delta H-f(m)theta with the atomic numbers of the elements in the lanthanide series was examined. The results show that a certain amount of covalence is present in the chemical bond between rare earth cations and the ligand. (C) 2009 Elsevier B.V. All rights reserved.