In this paper, the potentiometric method was carried out to investigate the thermodynamic properties of the RbF + Rb2SO4 + H2O and CsF + Cs2SO4 + H2O ternary systems over total ionic strengths from (0.01 -0.70) mol.kg(-1) for different ionic strength fractions y(B) of Rb2SO4/Cs2SO4 with y(B) = 0.00, 0.30, 0.60 and 0.90, at 298.2 K. The Hulled model and the Pitzer model were used to correlate the experimental data. The Harned coefficients and the Pitzer mixing parameters were evaluated by multiple linear regression technique. Moreover, the mean ionic activity coefficients of RbF/CsF and Rb2SO4/Cs2SO4, the osmotic coefficients, and the excess Gibbs energies of the studied systems were calculated, respectively. (C) 2016 Elsevier B.V. All rights reserved.