Measurements of isochoric heat capacities C-V for pure water and C-V,C-x for (H2O + KNO3) as a function of temperature along the different isochores and on the coexistence curves are reported. The measurements cover the range in temperature from 299 K to 536 K at x(KNO3) = 0.0017969. For pure water, measurements have been performed in the temperature range 321 K to 576 K at densities from 712 kg . m(-3) to 990 kg . m(-3). The experiments were performed at six isochores : (793.85, 816.22, 883.96, 932.70, 978.20, and 1002.40) kg . m(-3) for (H2O + KNO3), and seven isochores : (712.66, 799.22, 864.83, 916.31, 938.79, 956.76, and 989.58) for pure water. The measurements were taken 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 1.5 per cent. One- and two-phase heat capacities, temperatures, and densities at saturation were obtained by the method using quasi-static thermograms. The isochoric heat capacities C-V,C-x are described with an equation of state originally proposed for NaCl by Pitzer and co-workers, and the deviations in fit are +/-2 per cent or less.