The first part of this work deals with the determination of the thermodynamic parameters for the complexation of lanthanide cations (La3+, Pr3+ and Nd3+) with Cryptand-221 and Cryptand-222 in acetonitrile and propylene carbonate at 298.15 K. The complexation process between these cations and these ligands in these solvents is enthalpy-controlled. The higher stability observed for these cations and these ligands in propylene carbonate with respect to acetonitrile is attributed to the increase in entropy observed for the complexation reaction in propylene carbonate. Enthalpies of solution of lanthanide and lanthanide cryptates are reported. These data are used to derive single-ion enthalpies of transfer of La3+, Pr3+ and Nd3+ from propylene carbonate to acetonitrile based on the Ph4AsPh4B convention. The results show that the cryptate conventions are not valid for the calculation of single-ion values for the transfer of tervalent lanthanide cations among dipolar aprotic media. Enthalpies of coordination of lanthanide(III) cryptates in the solid state are calculated. The second part of this study aims to investigate the properties of the synthetic macrocyclic ligands such as Cryptand-222 and 18-Crown-6 towards molecules of biological importance. Stability constants (hence free energies), enthalpies and entropies of complexation of a series of DL-amino acids with 18-Crown-6 and Cryptand-222 in methanol and ethanol, as obtained from titration calorimetry, are reported. No significant variations are found in the free energies of complexation of the different amino acids and these two ligands in these solvents as a result of an enthalpy-entropy compensation effect. This effect is for the first time shown in complexation reactions involving crown ethers and cryptands. The thermodynamic parameters of transfer of amino acids and their complexes with 18-Crown-6 and Cryptand-222 from methanol to ethanol have been calculated. Possible correlations between complexation and transfer data for the amino acids, the ligands and the amino acid-macrocyclic ligand complexes are investigated. The implications of these results to processes of biological importance are discussed. As a continuation of this study, the possibility of selectively extracting amino acids from methanol by polymeric resins containing crown ethers as anchor groups is investigated.