Isopiestic vapor-pressure measurements were made for Rb2SO4(aq) from molality m = (0.16886 to 1.5679) mol (.) kg(-1) at T = 298.15 K and from m (0.32902 to 1.2282) mol (.) kg(-1) at T = 323.15 K. and for Cs2SO4(aq) from m = (0.11213 to 3.10815) mol (.) kg(-1) at T = 298.15 K and from m = (0. 11872 to 3.5095) mol (.) 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 molalitics 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 RbSO4(aq) and Cs2SO4(aq) for the more restricted range of m less than or equal to 0.101 mol (.) kg(-1). Values of the thermodynamic solubility product K-s(Rb2SO4, cr, 298.15 K) = (0.1392 +/- 0.0154) and the CODATA compatible standard molar Gibbs free energy of formation Delta(f)G(m)(o)(Rb2SO4, 298.15 K) = -(1316.91 +/- 0.59) kJ (.) mol(-1), standard molar enthalpy of formation Delta(f)H(m)(o)((RbSO4)-S-2, cr, 298-15 K) = -(1435.07 +/-0.60) kJ (.) mol(-1), and standard molar entropy S-m(o) (Rb2SO4, cr, 298.15 K) = (199.60 +/- 2.88) J (.) K-1 (.) mol(-1) were derived. A sample of one of the lots of Rb2SO4(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 Cs2SO4(s) used for some of our experiments showed it was highly pure.