Isopiestic vapor pressure measurements were made for {zH 2 SO 4 + (1 −z)MgSO 4 } (aq) solutions with H 2 SO 4 molality fractions ofz= (0.43040, 0.28758, and 0.14399) at the temperature 298.15 K. Measurements extend from total molalities ofm T = (0.29999, 0.32948, and 0.36485) mol· kg −1 tom T = (4.8474 0 , 4.2843 3 , and 3.9620 6 ) mol· kg −1 , respectively. These highest molalities correspond to the greatest degrees of supersaturation that could be achieved during isothermal removal of solvent. The corresponding water activity ranges are 0.9924 ≥a w ≥ 0.7469, 0.9924 ≥a w ≥ 0.7983, and 0.9924 ≥a w ≥ 0.8291 forz= (0.43040, 0.28758, and 0.14399), respectively. Isopiestic results for this system withz= (0.85811, 0.71539, and 0.57353) were reported previously (J. Chem. Thermodynamics1997, 29, 533–555). The combined results tom T ≤ 3.5 mol· kg −1 are represented essentially to experimental accuracy with Pitzer's model, using ionic strength-dependent third virial coefficients for the constituent binary solutions. A second set of parameters is also presented for the more extended molality range ofm T ≤ 6.0 mol· kg −1 , where this model gives a slightly less accurate representation of the experimental information. The nature and extent of ionic association in MgSO 4 (aq) is discussed, but it was not possible to select a value of the association constant owing to large discrepancies among the various studies.