Abstract A brief review of the present state of knowledge of the structure of water and aqueous electrolyte solutions is given. Interactions between water and the main group of ions are recognized to be of a similar order of magnitude as water—water interactions. As a consequence of ion hydration consisting of a group of water molecules being attached to the ions, pores in desalination membranes should be big enough to dissolve water but small enough not to dissolve this group of water molecules. Computer simulation of NaClO 4 solutions indicates the existence of outer sphere complexes anion anionH 2Ocation, which would also require the membranes to have larger pores. Since the series of ion rejection is parallel to the lyotropic ion series, it could be recognized by I.R. spectra as a series of different interactions to water. The water Hbonds to cellulose membranes are recognized as weaker than in water, but the adsorption heat of hydrate water is higher as a result of van der Waals contributions and changes in the membrane OH Hbonds. A mechanism of non-freezable water is given.