Pabalan and Pitzer (Geochim. Cosmochim. Acta 1.988, 52, 2393-2404) reported a comprehensive set of thermodynamic properties of aqueous Solutions of sodium sulfate without using ion association or hydrolysis. However. there is now ample evidence available indicating that the ion association cannot be ignored at temperatures T >= 373 K. For example, even at the lowest concentration of their studies (m >= 0.05) and at 573.15 K, less than 20% Of SO42-(aq) is available as free ions. Tit the present study, the integral heats of solution of sodium Sulfate were measured to very low concentrations (10(-4) m) up to 573.16 K. The data were analyzed correcting for the hydrolysis Of SO42-(aq) and the association of Na+(aq) with SO42-(aq) and NaSO4-(aq) in order to obtain the final standard state thermodynamic properties of completely ionized aqueous sodium sulfate, Na2SO4(aq). From these and the available solubility data, the stoichiometric activity coefficients of saturated aqueous Solutions of sodium sulfate were calculated up to 573.15 K and compared with literature data. The stoichiometric activity coefficients of aqueous solutions of sodium sulfate, as a function of temperature at all concentrations (0 <= m <= m(sat)), were also calculated up to 573.15 K.