Isopiestic vapor-pressure measurements were made for Na2SO4(aq) from 0.11 to 3.74 mol-kg-1 at 25.00°C and from 0.12 to 3.57 mol-kg-1 at 50.00°C, using NaCl(aq) as the reference standard. Published isopiestic data, direct vapor pressures, emfs of reversible cells, freezing temperature depressions, boiling temperature elevations, heat capacities, and dilution enthalpies for this system have been critically assessed and recalculated in a consistent manner. Parameters for Pitzer's model and a mole fraction-based thermodynamic model were evaluated. The mole fraction-based thermodynamic model for Na2SO4(aq) is valid from the freezing temperatures of the solutions to 150.5°C. Pitzer's model represents the osmotic coefficients and emfs essentially to experimental accuracy at 25 and 50°C provided that the third virial coefficient was ionic-strength dependent.