Isopiestic vapor pressure measurements were made for {yMgCl 2 + (1 -y)MgSO 4 }(aq) solutions with MgCl 2 ionic strength fractions of y = (0, 0.1997, 0.3989, 0.5992, 0.8008, and 1) at the temperature 298.15 K, using KCl(aq) as the reference standard.These measurements for the mixtures cover the ionic strength range I = 0.9794 to 9.4318 mol·kg -1 . In addition, isopiestic measurements were made with NaCl(aq) as reference