This paper reports the synthesis and thermodynamic properties of three rare earth complexes, which were synthesized from praseodymium nitrate hexahydrate (Pr(NO3)(3)center dot 6H(2)O) and heterocyclic ligands of niacin (vitamin PP, C6H5NO2) or 8-hydroxylquinoline (C9H7NO). Their compositions and structures were characterized by elemental analysis, molar conductance, thermogravimetric analysis, UV-visible spectroscopy, and IR spectroscopy. The ligand C6H5NO2 was bidentate-coordinated with Pr3+ ions through an oxygen atom of its carboxylic group which was formed by removing the proton from the hydroxyl group: the heterocyclic nitrogen atom from C9H7NO- formed a chelating ring around with Pr3+ ion. At a constant temperature of 298.15 K, the dissolution enthalpies of the reactants and products of the coordination reactions in the optimized calorimetric solvent were determined by an advanced solution-reaction isoperibol microcalorimeter. The standard molar enthalpies of the coordination reactions were determined. By combination of the experimental values of enthalpies of dissolution with some other auxiliary thermodynamic data through a designed thermochemical cycle on the basis of a supposed chemical reaction, the standard molar enthalpies of formation of the synthetic coordination complexes were estimated to be Delta H-f(m)Theta[Pr(C6H4NO2)(2)C9H6NO(s), 298.15 = K] = -(1448.3 +/- 2.6) kJ.mol(-1), Delta H-f(m)Theta[Pr(C6H4NO3)(3)center dot 2H(2)O(s), 298.15 K] = -(2256.1 +/- 2.9) kJ.mol(-1), and Delta H-f(m)Theta[Pr(C9H6NO)(3)center dot 4H(2)O(s), 298.15 K] = -(1975.3 +/- 4.6) kJ.mol(-1).