A model, based on Helmholtz energy, that can be used to describe thermodynamics of NaCl solutions in the water near-critical region has been developed and tested as a function of temperature (350-400 degrees C), pressure (18-41 MPa), and solute concentration (0-5 molal) using literature densities, apparent NaCl (aq) molar volumes, heats of dilution, and apparent isobaric molar heat capacities. Densities, heats of dilution, apparent molar isobaric heat capacities, and apparent NaCl (aq) molar volumes calculated using the model agree well with literature values over the conditions studied. Calculated apparent molar isobaric heat capacities agree with literature data up to 450 degrees C, even though the data used in model regression were valid only at temperatures from 350 to 400 degrees C. Since calculation of apparent molar isobaric heat capacities involves the second temperature derivative of Helmholtz energy, extrapolation to temperatures > 400 degrees C is expected to give reasonable results.