Solution-reaction calorimetry was used to determine the standard molar (p'=0.10 Mpa) enthalpies of formation of crystalline chelates Ln(thd)(3) (Ln=Nd, Sm, Gd, Tb, Er, Tm, Yb, Lu and thd=2,2,6,6-tetramethyl-3,5-heptanedionate), at (T=298.15+/-0.02) K, to give: 4054+/-1.21; 37.77+/-1.78; 8.06+/-0.80; 26.43+/-1.61; 51.05+/-1.10; 66.75+/-1.30; 50.39+/-1.64 and 39.00+/-1.33 kJ mol(-1), respectively. The corresponding molar enthalpies of sublimation 159.1+/-3.4; 149.7+/-3.4; 166.1+/-3.5; 138.4+/-2.6; 130.8+/-2.2; 131.3+/-2.9; 131.1+/-2.7 and 135.8+/-2.9 kJ mol(-1) were obtained by means of differential scanning calorimetry. From the standard molar enthalpies of dissociation of the chelates 1518+/-62; 1405+/-62; 1605+/-62; 1599+/-62; 1525+/-62; 1408+/-62; 1333+/-62; and 1720+/-62 kJ mol(-1), respectively, the mean lanthanide-oxygen bond dissociation enthalpies were also calculated to be 253+/-10; 234+/-10; 268+/-10; 266+/-10; 254+/-10; 235+/-10; 222+/-10 and 287+/-10 kJ mol(-1), by assuming the equivalence of all bonds in the chelates. This last value correlated with the atomic number of the metal elements to give an inclined W, proposed to systematize lanthanide properties, also indicating that the nature of the coordination established on lanthanide is very close to that found in the respective lanthanide oxides. (C) 1999 Elsevier Science Ltd. All rights reserved.187969977