A new model is reported that combines a Helmholtz energy expression developed by Anderko and Fitter and is valid in the critical region of water with a mathematical equation developed by Wood and co-workers describing the dissociation of aqueous sodium chloride. The new model takes into account the effects of ionic dissociation, hard-sphere interactions, and dipole-dipole interactions. The new model and the Anderko-Pitzer model were tested by comparing measured heats of dilution of NaCl in the near-critical and supercritical regions of water with those calculated using these models. The heats of dilution of aqueous NaCl solutions calculated using the Anderko-Pitzer model were in poor agreement in the dilute concentration region with these reported by Busey et al. and Chen et al. and with new results reported here. The agreement between experimental heats of dilution and those calculated using the Anderko-Pitzer model depended on the variables solute concentration, temperature, and pressure. This agreement improved as the solute concentration or temperature increased or as the pressure decreased, while the other two variables remained constant. The changes in each case resulted in greater ion association. The incorporation of a species dissociation term into the Anderko-Pitzer model gave improved results in dilute solutions, at lower temperatures, and at higher pressures where ion association is least.