Isopiestic vapor pressure measurements were made for {ZH2SO4 + (1 - z)MgSO4}(aq) solutions with H2SO4 molality fractions of z = (0.43040, 0.28758, and 0.14399) at the temperature 298.15 K. Measurements extend from total molalities of m(T) = (0.29999, 0.32948, and 0.36485) mol kg(-1) to m(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 greater than or equal to a(w) 0.7469, 0.9924 greater than or equal to a(w) greater than or equal to 0.7983, and 0.9924 greater than or equal to a(w) greater than or equal to 0.8291 for z = (0.43040, 0.28758, and 0.14399), respectively. Isopiestic results for this system with z = (0.85811, 0.71539, and 0.57353) were reported previously (J. Chem. Thermodynamics 1997, 29, 533-555). The combined results to mT less than or equal to 3.5 mol.kg(-1) are represented essentially to experimental accuracy with Fitter'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 of m(T) less than or equal to 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 MgSO4(aq) is discussed, but it was not possible to select a value of the association constant owing to large discrepancies among the various studies.