Isopiestic vapor-pressure measurements were made for Rb 2SO 4(aq) from molalitym = (0.16886 to 1.5679 )mol · kg - 1atT = 298.15 K and from m = (0.32902 to 1.2282 )mol · kg - 1at T = 323.15 K, and for Cs 2SO4 (aq) from m = (0.11213 to 3.10815 )mol · kg - 1at T = 298.15 K and fromm = (0.11872 to 3.5095 )mol · kg - 1atT = 323.15 K, with NaCl(aq) as the reference standard. Published thermodynamic information for these systems were reviewed and the isopiestic equilibrium molalities and dilution enthalpies were critically assessed and recalculated in a consistent manner. Values of the four parameters of an extended version of Pitzer‘s model for osmotic and activity coefficients with an ionic-strength dependent third virial coefficient were evaluated for both systems at both temperatures, as were those of the usual three-parameter Pitzer model. Similarly, parameters of Pitzer‘s model for the relative apparent molar enthalpies of dilution were evaluated at T = 298.15 K for both Rb 2SO 4(aq) and Cs 2SO 4(aq) for the more restricted range of m 0.101 mol · kg - 1. Values of the thermodynamic solubility product Ks(Rb2 SO 4, cr, 298.15 K ) = (0.1392 ± 0.0154) and the CODATA compatible standard molar Gibbs free energy of formation?fGmo (Rb 2SO 4, cr, 298.15 K ) = - (1316.91 ± 0.59)kJ · mol - 1, standard molar enthalpy of formation?fHmo (Rb 2SO 4, cr, 298.15 K ) = - (1435.07 ± 0.60)kJ · mol - 1, and standard molar entropy S mo(Rb2 SO 4, cr, 298.15 K ) = (199.60 ± 2.88)J · K - 1· mol - 1were derived. A sample of one of the lots of Rb 2SO 4(s) used for part of our isopiestic measurements was analyzed by ion chromatography, and was found to be contaminated with potassium and cesium in amounts that significantly exceeded the claims of the supplier. In contrast, analysis by ion chromatography of a lot of Cs 2SO 4(s) used for some of our experiments showed it was highly pure.