The standard partial molar volumes, V-infinity(i,T), of 12 univalent ions (alkali metal, ammonium, halide, nitrate, and perchlorate) and five divalent ions (alkaline earth and sulfate) in water at 125, 150, 175, and 200 degrees C and at 2 MPa were derived from the data of Ellis. Similar data for NH4+ and NO3- at 0-100 degrees C, not included in Part 4, were added, derived from his data too. The (negative) electrostrictive volumes, Delta V-elstr(i,T), of these ions at infinite dilution were obtained from the shell-by-shell calculation of the electrostriction according to Marcus and Hefter that takes into account the mutual dependence of the relative permittivity of the water around the ion and the electrical field strength at it. The expanded volumes of the ions, defined as V-Expan(i,T) = V-infinity(i,T) - Delta V-elstr (i,T), were then derived and compared with their intrinsic volumes, calculated according to Glueckauf. The calculation yields also the spatial extension of the dielectrically saturated region around the ions. The numbers of water molecules, the molar volume of which is affected by the ions at infinite dilution, were estimated from the ratio of Delta V-elstr(i,T) and V-wel(T), the latter being the average molar electrostriction of the water in the vicinity of the ions.