Résumé : The interactions of 18-crown-6, 15-crown-5m and 12-crown-4 with Na** plus and K** plus were studied in methanol and water as solvents at 25 degree C. DELTA G degree values for both 1:1 and 2:1 complexation reactions were determined by potentiometric titrations. Used in conjunction with these values, calorimetric measurements led to DELTA H degree and DELTA S degree values. The thermodynamic parameters obtained cannot be correlated with the cations or the crown ethers ″hole″ sizes in any 1:1 or 2:1 reactions. Moreover, the DELTA G degree values are the result of quite different but permanently compensating combinations of the DELTA H degree and DELTA S degree values. These arise from several thermodynamic processes in which the role of the solvent must be considered. In the case of 18-crown-6, the authors present a quantitative interpretation in which this crown ether develops interactions that are stronger with Na** plus than with K** plus .