Isopiestic vapor-pressure measurements were made for Na2SO4(aq) from 0.11 to3.74 mol-kg−1 at 25.00°C and from 0.12 to 3.57 mol-kg−1 at 50.00°C, usingNaCl(aq) as the reference standard. Published isopiestic data, direct vaporpressures, emfs of reversible cells, freezing temperature depressions, boilingtemperature elevations, heat capacities, and dilution enthalpies for this system have beencritically assessed and recalculated in a consistent manner. Parameters for Pitzer'smodel and a mole fraction-based thermodynamic model were evaluated. Themole fraction-based thermodynamic model for Na2SO4(aq) is valid from thefreezing temperatures of the solutions to 150.5°C. Pitzer's model represents theosmotic coefficients and emfs essentially to experimental accuracy at 25 and50°C provided that the third virial coefficient was ionic-strength dependent.