Activity coefficients for the CsCl + Cs2SO4 + H2O ternary system were determined at 298.15 K by electromotive force (EMF) measurements of the cell: Cs ion-selective electrode (ISE)|CSCl(m(A)), Cs2SO4(m(B)), H2O|Ag|AgCl. The cesium ion-selective electrode (Cs-ISE) and the Ag|AgCl electrode used in this work were made in this laboratory and had a good Nernst response. The experimental data were interpreted using Hamed's rule and the Pitzer model. The corresponding parameters required in the Pitzer model have been evaluated for this system. Osmotic coefficients of this system and activity coefficients of the second salt Cs2SO4 were also calculated in the same molality range.