This paper discusses the computer simulation of the diffusion coefficients and structure of infinitely dilute aqueous ionic solutions at ambient (298 K, solvent density 0.997 g cm(-3)) and supercritical (683 K, solvent density 0.35 g cm(-3)) conditions using two different models for water. They are the extended simple point charge (SPC/E) and renormalized polarizability (RPOL) models in which the electronic polarizations of the water molecule are treated differently. The effect of polarizability is implicit in the SPC/E model and explicit in the RPOL model. The RPOL model shows slightly greater hydrogen bonding, at room temperature than the SPC/E model, but less hydrogen bonding at 683 K. It is concluded that the explicit neglect of electronic polarization of the solvent and ions in calculations based on the SPC/E model of water, has only a small effect on the diffusion coefficients of the ions. Both models predict diffusion coefficients of ions in supercritical water that are weakly dependent on their size in contrast to their behavior under ambient conditions discussed in previous work [S. Koneshan , J. Phys. Chem. 102, 4193 (1998)]. The simulations suggest that the mechanism of diffusion at the ambient and supercritical states of the solvent water is different.