The thermodynamic parameters for the complexation reaction of Sr(II) and Ba(II) with 18 Crown 6 (18C6) were determined in the binary water-ethanol mixtures as solvent. The stability of both complexes was found to increase with increase in ethanol mole fraction (X-E). The complexation reaction is exothermic with the Delta H value increasing with X-E. The entropy of complexation was found to decrease for XE UP to 0.1 above which it increased monotonically. The complexation process was enthalpically favored but entropically unfavorable. The data have been explained in terms of the solvent effects on metal ion, ligand and complex stabilization in binary mixture. The ab initio calculations were also carried out to investigate the stability of metal ion and ligand in water and ethanol medium. (C) 2009 Elsevier B.V. All rights reserved.