Vapor pressures of 1.0-6.1 mol kg™1 aqueous sodium chloride were measured from 298 to 373K. The apparatus for measuring vapor pressures was modified to increase the convenience and the precision so that it could be operated by a single observer; the precision of measurement was ±0.002K or ±0.005 torr, whichever was larger. From these measurements, other vaporpressure and freezing-point measurements, and calorimetric enthalpies and heat capacities at 298K, 20 parameters of the modified Debye-Huckel-powerseries-in-the-molality equation were determined for aqueous sodium chloride. The equation was compared with measurements of the vapor pressure, the electromotive force of concentration cells, and thermal properties. The agreement was usually excellent.Previous static measurements of liquid-vapor equilibrium (33-35) indicated that the method is capable of even greater precision. Therefore, we reconstructed the cell and most of the auxiliary apparatus (9, 10). The changes which are of general interest were the replacement of the membrane null manometer by a liquid mercury manometer and the replacement of the air thermostat by an oil thermostat. The quantities to be measured were the equilibrium temperature and pressure, the total amount of each component, and the volume of the vapor phase.