Apparent molar volumes, V-phi,V-2, of aqueous NaCl, NaOH, NaOD, HCl, and DCl in water and heavy water were determined at T = 523 and 573 K and p = 14 MPa with a high-temperature platinum vibrating-tube densimeter in the aquamolality range 0.25 <= m(aq) <= 2.5 mol center dot (55.509 mol solvent)(-1). The experimental results have been represented with an extended Debye-HUckel equation to describe the concentration dependence of V-phi,V-2 and to derive standard partial molar volumes of these electrolytes in light and heavy water, V degrees(2,H) and V degrees(2,D), respectively. For NaCl and NaOH, the D2O isotope effect at infinite dilution, [V degrees(2,H) - V degrees(2,D)], increases from 0.2 and 0.8 cm(3) mol(-1) to 4.5 and 7.1 cm(3) mol(-1), respectively, when the temperature is increased from 523 to 573 K. For HCl and DCl, the effect is smaller and the sign is reversed, [V degrees(2,H) - V degrees(2,D)] = -0.7 cm(3) mol(-1) at 523 K and -1.4 cm(3) mol(-1) at 573 K. When the effect of ion association is included, the deuterium isotope effect for HCl becomes positive, [V degrees(2,H) - V degrees(2,D)] approximate to 17 cm(3) mol(-1) at 573 K, consistent with NaCl and NaOH. Two models are proposed to describe the solvent isotope effect on the infinite dilution limit, one based on the Born equation and the other on the dimensionless Krichevskii parameter(.) The experimental values of V degrees(2,D) also have been used to calculate the first reported values for the pressure dependence of the ionization constant of D2O at temperatures higher than 313 K.