Isopiestic vapor-pressure measurements have been performed for aqueous solutions of well-characterized high-purity NaBr from 1.9551 to 9.4778 mol.kg(-1) at 298.15 K; the highest five molalities correspond to supersaturated concentrations. Solubilities have also been determined by this method. A few equilibrations were made between solutions of NaCl and H2SO4 to refine the osmotic coefficients of H2SO4 at high molalities. These isopiestic results for NaBr have been combined with other experimental thermodynamic quantities (vapor pressures, activity coefficients, solubilities, freezing temperatures, and volumetric and calorimetric measurements) to yield revised parameters for an extended form of Fitter’s equation applicable over wide ranges of molality, temperature, and pressure. It was not possible to obtain a complete consistency between the experimental results for either the NaBr + H2O system or the NaCl + H2O system with entropies from the CODATA Key Values for Thermodynamics.