Changes of structure of the water solvent caused by organic and inorganic ions have been investigated by measurements of the H₂O-D₂O solvent isotope effect in the partial molar volumes V₂ of alkali halides, tetraalkylammonium (TAA) and other ammonium type salts over a range of temperatures. The structure promoting hydrophobic ions have larger V₂ in D₂O than in H₂O, while the more electrostricting simple inorganic ions have smaller volumes in D₂O than in H₂O. By means of an extrapolation procedure based on the additivity of V°₂ for homologous TAA salts, the individual ionic contributions to the solvent isotopic volume difference can be evaluated. On this basis, the results indicate that the anions are predominantly the species that are associated with the so-called structure breaking effects in the simple salt solutions while K⁺ and the TAA cations exhibit an opposite kind of behaviour . The solvent isotope effect in V₂ for pyridine and H (as HOD) is also evaluated. Studies over a range of temperature enabled the partial molal expansibilities to be evaluated in H₂O and D₂O.