The combustion energies have been determined for fifteen solid complexes of rare-earth nitrate with histidine. The standard enthalpies of combustion, Delta(f,coor)H(theta), and standard enthalpies of formation, Delta(f,coor)H(theta) have been calculated for these complexes. The relationship of Delta(f,coor)H(theta) with the atomic numbers of the elements in the lanthanide series has been examined. The results show that a certain amount of covalence is present in the chemical bond between the rare-earth cations and histidine.