The complex formation of 18-crown-6 with some alkali, alkaline earth, and ammonium ions in dioxane-water mixture has been studied by means of calorimetric titrations. The stability constants, reaction enthalpies and entropies for the complex formation in mixtures of water and dioxane at various proportions have been measured. The results clearly indicate the preferential solvation of the alkali and alkaline earth cations by water molecules. Thus, the values of the reaction entropies for a given cation are independent of the solvent composition. In the case of ammonium ions, the formation of hydrogen bonds between the NH-groups of the ammonium and mono and di-substituted ammonium cations and the water molecules has a pronounced influence upon the thermodynamic values measured. The results strongly demonstrate the influence of solvent composition and of selective solvation of cations in dioxane-water mixtures on the complex formation of 18-crown-6 with cations. (C) 2008 Elsevier B.V. All rights reserved.