We report a new method for determining the saturation vapor pressure, P-s(T), of water at conditions below its triple point. P-s(T) appears as a parameter in the statistical rate theory (SRT) expression for the net evaporation flux. We use measurements of the interfacial conditions during steady-state evaporation and condensation experiments and SRT to determine the values of P-s(T) from 50 different experiments over a range of interfacial conditions. From these values of P-s(T), we develop an analytical expression and from it calculate the liquid-vapor latent heat, L-1v(T), and the constant pressure specific heat, c(p)(L)(T). The calculated values of these properties are compared with those obtained from independent measurements. This comparison indicates the SRT expressions for L-1v(T) and c(p)(L)(T) are consistent with the measurements over a range of temperatures.