The heat capacity at constant volume C-VX of aqueous solutions of Na,SO, has been measured from 460 to 669 K along 11 isochores from 250 to 926 kg m(-3) for three compositions, namely : 0.0012798; 0.0066328, and 0.0139002 mole fraction of Na,SO,. Measurements of the isochoric heat capacity C-V for pure water were also made at the critical isochore(rho(c) = 317.9 kg m(-3)) and at six near-critical isochores, namely : 309.598, 335.570, 354.480, 368.310, 370.370 and 373.340 kg m(-3), and the coexistence curve. A total of 368 C-VX data for H2O + Na2SO4 and 273 C-V data for pure water were measured along 17 isochores in the one- and two-phase regions. The measurements were made in a high-temperature and high-pressure adiabatic nearly constant-volume calorimeter. The uncertainty of the isochoric heat capacity measurements is estimated to be within +/-2%. One-(C＇(V)) and two-phase (C"(V)) heat capacities, temperatures (T-S), and densities (rho(S)) at saturation were obtained by the method of quasi-static thermograms. The experimental C-VX results have been compared with Fitter＇s classical equation of state. The relative average deviations for H2O + Na2SO4 solutions were about +/-2%, except for the critical region where differences reached 15-20%. (C) 1998 Elsevier Science B.V. All rights reserved.