Isopiestic vapor-pressure measurements were made for {zH(2)SO(4) + (1 - z)MgSO4}(aq) with H2SO4 molality fractions of z = (0.85811, 0.71539, and 0.57353) at the temperature 298.15 K. Measurements extend from total molalities m(T) of (0.12548, 0.13563, and 0.14741) mol.kg(-1) to the supersaturated molalities m(T) = (12.049(6), 11.011(2), and 7.2059(5)) mol.kg(-1), respectively. For z =(0.71539 and 0.57353) these highest molalities correspond to the greatest degrees of supersaturation that could be achieved during isothermal removal of the solvent. The corresponding water activity ranges are 0.9958 greater than or equal to a(w) greater than or equal to 0.2758, 0.9958 greater than or equal to a(w) greater than or equal to 0.3312, and 0.9958 greater than or equal to a(w) greater than or equal to 0.5581 for z = (0.85811, 0.71539, and 0.57353), respectively. These values of z were chosen to be separated approximately equally by 0.142 (Delta z approximate to 1/7). The water activity curves were also found to be nearly equally spaced from each other, as was the z = 0.85811 curve from that for H2SO4(aq) (z = 1). Given that both H2SO4(aq) and MgSO4(aq) are extensively associated electrolytes, this regularity in the mixing behavior was not anticipated. A well-defined crossover point was found at which the sums of the stoichiometric ionic molalities were equal at isopiestic equilibrium for z = (1,0.85811, 0.71539, and 0.57353), as were the water activities and stoichiometric osmotic coefficients of these solutions. This occurred at Sigma(i) nu(i)m(i) = (5.9632 +/- 0.0036) mol.kg(-1), where nu(i) is the stoichiometric ionization number of electrolyte i.