The enthalpies of transfer of Co(II) ion in several aqueous-organic and organic mixed solvents were analysed by means of the theory of preferential solvation. The results provided the quantitative description of preferential solvation of the ion and of its effect on interactions between solvent molecules. They also enabled to compare solvating properties of various mixtures. The analysis revealed a discontinuity in those properties with the composition of the solvent, caused by the changes in the solvent structure. The mean number of molecules of one component of the mixture in the solvation sphere of Co(H) ion versus solvent composition was determined. (C) 2004 Elsevier B.V. All rights reserved.