The mean activity coefficients of the CsCl + propylene glycol + water system were determined at 298.15 K using potentiometric measurements. The range of CsCl molality (m) is (0.01 to 1.20) mol.kg(-1), and the mass fraction of propylene glycol (w) in mixed solvent is 0.10 to 0.40. The Pitzer, the modified Pitzer, and the extended Debye-Huckel equations are used to describe the nonideal behavior of the electrolyte. The activity coefficients and the osmotic coefficients of CsCl were calculated by the Pitzer equation. We also obtained the Gibbs energy of transfer of CsCl from water to propylene glycol + water mixtures.