A model, based on Helmholtz energy, describing the thermodynamics of NaCl solutions in the near-critical region of water has been developed and tested as a function of temperature (350402 degreesC), pressure (18-41 MPa), and solute concentration (0-5 m) using literature heat of dilution (Delta(dil)H) data. This model is an extension of one developed by Anderko and Pitzer (Geochim. Cosmochim. Acta 1993, 57, 1657-1680) as modified by Oscarson et al. (Ind. Eng. Chem. Res. 2001, 40, 2176-2182). The present model includes terms for NaCl dissociation and ion-ion interactions. Density and Delta(dil)H values predicted using the model agree well with literature values over the temperature, pressure, and solute concentration ranges of the study. Apparent log K', Delta(r)H, Delta(r)S, and Delta(r)C(p) values change significantly with the solute concentration, temperature, and solution density. The model was used to determine speciation in aqueous NaCl solutions under the conditions of the study.