The mean activity coefficients of the ternary systems CsCl + polyethylene glycol (PEG) (w) + H2O (1 - w) (w = 10%, 20%, 30%, 40%) determined by the potentiometric method at T = 298.15 K are reported. The molecular weights of PEG are 200 and 600, and the CsCl molality varies from 0.01 to 4.00 mol/kg. To correlate the experimental data, the Pitzer equation, extended Debye-Huckel equation, and the modified Pitzer equation have been utilized herein. As a result, a series of thermodynamic parameters were obtained and discussed in detail, including the mean ionic activity coefficients (gamma(+/-)), the osmotic coefficient (Phi), the excess Gibbs free energies (G(E)), the standard Gibbs free energy of transference (Delta G(t)(0)) and the primary hydration number (eta(hydr).).