Isopiestic vapor-pressure measurements were made for ZnSO4(aq) at T = 298.15 K, both at Texas Christian University and at Lawrence Livermore National Laboratory, from molality m = 0.1004 mol . kg(-1) to the supersaturated m = 4.3092 mol . kg(-1), and included solubility determinations. These results were combined with critically assessed e.m.f. results from m = 6.7 . 10(-4) to m = 3.4413 mol . kg(-1) to yield recommended values of the osmotic coefficients, water activities, and mean activity coefficients of ZnSO4(aq), along with the thermodynamic solubility product and a CODATA compatible value for the Gibbs free energy of formation of goslarite: K-s{ZnSO4 . 7H(2)O(cr, orthorhombic)} = (0.01119 +/- 0.00033) and Delta (f)G(m)(o),(ZnSO4 . 7H(2)O, cr, orthorhombic, 298.15 K) = -(2562.32 +/- 0.57) kJ . 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 ZnSO4(aq) undergo a change from negative deviations from the Debye-Huckel limiting law below m similar to 0.01 mol . kg(-1) to positive deviations at higher molalities as described by Malatesta and Zamboni (J. Solution Chem. 1997, 26, 791-815), and our recommended values were adjusted for this effect. The standard potential of the reversible electrochemical cell: ZnHgx(two phase)\ZnSO4(aq)\PbSO4(cr)\PbHgx(two phase) was evaluated, and the resulting value was used for evaluating the standard potentials of the PbSO4(cr)\PbHgx(two phase) and Hg2SO4(cr)\Hg(1) electrodes. (C) 2000 Academic Press.