Isopiestic vapor-pressure measurements were made for ZnSO 4 (aq) at T = 298.15 K, both at Texas Christian University and at Lawrence Livermore National Laboratory, from molality m = 0.1004 mol β’ kg -1 to the supersaturated m = 4.3092 mol β’ kg -1 , and included solubility determinations. These results were combined with critically assessed e.m.f. results from m = 6.7 β’ 10 -4 to m = 3.4413 mol β’ kg -1 to yield recommended values of the osmotic coefficients, water activities, and mean activity coefficients of ZnSO 4 (aq), along with the thermodynamic solubility product and a CODATA compatible value for the Gibbs free energy of formation of goslarite: K s {ZnSO 4 β’ 7H 2 O(cr, orthorhombic)} = (0.01119 Β± 0.00033) and f G o m (ZnSO 4 β’ 7H 2 O, cr, orthorhombic, 298.15 K) = -(2562.32 Β± 0.57) kJ β’ mol -1 . An equation of the type used by Pitzer and Mayorga (J. Solution Chem. 1974, 3, 539-546) accurately represents the experimental osmotic coefficients and e.m.f.s provided it is modified by making the third virial coefficient ionic-strength dependent and by adding a fourth virial term. However, the mean activity coefficients of ZnSO 4 (aq) undergo a change from negative deviations from the Debye-HΓΌckel limiting law below m βΌ 0.01 mol β’ kg -1 to positive deviations at higher a This document was prepared as an account of work sponsored by an agency of the United States Government.