In 1968, R.F. Platford reported the results from extensive isopiestic vapor-pressure measurements for the {zNaCl + (1 - z)Na2SO4} (aq) system at T = 298.15 K, using NaCl(aq) as the isopiestic reference standard [R.F. Platford, J. Chem. Eng. Data 13 (1968) 46-48]. However, only derived quantities were reported, and the experimental isopiestic equilibrium molalities were not given. The complete set of original isopiestic molalities from that study is tabulated in the present report. In addition, published thermodynamic information for this system is reviewed and the isopiestic equilibrium molalities, electromotive force measurements for five different types of electrochemical cells, and enthalpies of mixing from these other studies are critically assessed and recalculated consistently. These combined results are used to evaluate at T = 298.15 K the two mixing parameters of Pitzer's ion-interaction model, (S)theta(Cl, SO4) = (1.236 +/- 0.032(5)) .10(-2) kg . mol(-1) and (Na, Cl, SO4) = (1.808 +/- 0.086) . 10(-3) kg(2). mol(-2), and their temperature derivatives {partial derivative(S)theta(Cl, SO4)/partial derivativeT}(P) = (2.474 +/- 0.460) . 10(-5) kg . mol(-1) . K-1 and {partial derivativepsi(Na, Cl, SO4)/partial derivativeT}(P) = -(6.228 +/- 0.186) . 10(-5) kg(2) . mol(-2) - K-1. Also reported are parameters for an extended ion-interaction model for Na2SO4(aq), valid from T= (273.15 to 323.15) K, that were required for this mixed electrolyte solution analysis. (C) 2003 Elsevier Science Ltd. All rights reserved.