The heat capacity at constant volume Cv of pure water has been measured in the temperature range from 412 K to 693 K at 13 isochores between 250 kg . m(-3) and 925 kg . m(-3). Measurements cover the critical region and coexistence curve. Measurements have been made in both the one- and two-phase regions near the phase transition points. The measurements were made in a high-temperature, highpressure adiabatic nearly constant-volume calorimeter. The uncertainty of the heat capacity measurements is estimated to be within +/-2.5%. Liquid and vapor one- (C-V1＇, C-V1") and two-phase (C-V2＇, C-V2") heat capacities, temperatures (T-S), and densities tps) at saturation were obtained by the method of quasistatic thermograms. The parameters (c, T*, V*) of the simplified-perturbed-hard-chain-theory (SPHCT) equation of state have been optimized to allow calculations of heat capacities for water in the vapor and liquid phases. The relative average deviations for H2O were within about +/-4.5%, except in the critical region where differences reached 15-20% or more. The two-phase heat capacity data C-V2 were used to evaluate the second temperature derivatives of the vapor pressure and chemical potential. Values of the vapor pressure of water were calculated from C-V2 measurements using critical pressure and the temperature derivative of the saturation pressure at the critical point.