Isopiestic vapor-pressure measurements were made for Rb 2 SO 4 (aq) from molality m = (0.16886 to 1.5679) mol โข kg -1 at T = 298.15 K and from m = (0.32902 to 1.2282) mol โข kg -1 at T = 323.15 K, and for Cs 2 SO 4 (aq) from m = (0.11213 to 3.10815) mol โข kg -1 at T = 298.15 K and from m = (0.11872 to 3.5095) mol โข kg -1 at T = 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 threeparameter 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 2 SO 4 (aq) and Cs 2 SO 4 (aq) for the more restricted range of m 0.101 mol โข kg -1 . Values of the thermodynamic solubility product K s (Rb 2 SO 4 , cr, 298.15 K) = (0.1392 ยฑ 0.0154) and the CODATA compatible standard molar Gibbs free energy of formation f G o m (Rb 2 SO 4 , cr, 298.15 K) = -(1316.91 ยฑ 0.59) kJ โข mol -1 , standard molar enthalpy of formation