Limiting partial molar volumes of electrolytes in several water-dimethyl sulfoxide mixtures, up to xOMSO = 0.1587, have been determined from density measurements at 298.15 K. A reference electrolyte assumption (TATB) has been used in order to obtain single-ion partial molar volumes of transfer from water to DMSO-water mixtures. The influence of DMSO on AtVi0 of alkali metals and halide ions is negligible in the highly water-rich region (up to xDMSO z 0.04) except for the Iion. However, when the DMSO composition is greater than xDMSO x 0.04, the variation of A,VI0 for alkali metals is the reverse to that of halide ions. The results are discussed on the basis of ion-solvent and solvent-solvent interactions using Conway's model.Studies of the partial molar volume of electrolytes at infinite dilution, Via, have proved to be useful for examining ionsolvent and solvent-solvent interactions in aqueous,' non-Studying the effects of solvents on electrolytes is a suitable way to show how the AI V;" values change with solvent composition. These changes are usually expressed by means of a transfer function, At V;", defined as and mixed aqueous solvents.8-'where w represents a reference solvent (water), and s the solvent or mixed solvent.The influence of DMSO on water structure is the subject of disagreement and the point of view adopted depends on the experimental procedure used (see Table 1 in ref. 13). Nevertheless, it seems that within the whole composition range these mixtures can be divided into three regions: one belonging to a highly water-rich region with a quasi-clathrate structure, another region up to xDMSO z 0.03 where associates, chiefly of the 2 H 2 0 -lDMSO type, are predominant, and a third governed by a dipolar structure based on dipole4ipole interaction^.'^,' Standard partial molar free energies, At G", and enthalpies, At H", of transfer, for several electrolytes have been studied in the entire range of DMSO composition.'6 However, we think that a non-energetic approach, such as Via, may contribute to