Measurements of isochoric heat capacities C-VX for pure water and aqueous salt solutions (H2O + NaCl, H2O + KCl and H2O + NaOH) as a function of temperature along the critical isochores and on the coexistance curves for different concentrations are reported. For binary system H2O + KCl, measurements of C-VX along four isochores (333.11, 368.32, 415.97, and 598.09 kg m(-3)) near the critical point of pure water are reported. Measurements of isochoric heat capacity C-V for pure water were also made at critical isochore (rho(c) = 317.9 kg m(-3)) and at five near-critical isochores, namely : 309.598, 335.570, 354.480, 370.370, and 373.340 kg m(-3), and the coexistence curve. The measurements cover the range in temperature from 371 to 827 K at concentrations from 0 to 0.0932 mole fraction of salt (KCl, NaCl and NaOH). The measurements were made in a high-temperature and high pressure adiabatic nearly constant-volume calorimeter. Uncertainties of the isochoric heat capacity measurements are estimated to be less than +/-2%. One-and two-phase heat capacities (C-VX', C-VX"), temperatures (T-s), and densities (rho(s)) at saturation were obtained by the method of quasi-static thermograms. The experimental C-VX results has been compared with a Fitter's classical equation of state and with the results of our previous measurements. The relative average deviations for H2O + NaCl, H2O + KCl and H2O + NaOH solutions within about +/-2%, except critical region where deviations reached 15-20%.