The experimental information on the pVT properties of aqueous electrolyte solutions over a wide range of temperature and pressure, which include the near critical region, is reviewed. General trends in the behaviour of the excess volumes are discussed. A theoretical expressions for the excess volume based on the mean spherical approximation (MSA) is described, which is the natural extension of the Debye-Huckel limiting law. The application of this approach to real systems shows that the MSA expression for the excess volume is valid up to moderate concentrated solutions. Only one adjustable parameter, the mean diameter of the ions, is required to fit the experimental data with an accuracy similar to the Pitzer ion-interaction model. Finally, the effect of ion association on the excess volume is evaluated for some simple systems. It is concluded that the ion-pair formation must be included in the data treatment at high temperature and low density region in order to obtain reliable partial molar volumes of electrolyte at infinite dilution.