This work reports the results of a thermodynamic investigation of the ternary CSCl-MgCl2-H2O mixed electrolyte system. The activity coefficients of this mixed aqueous electrolyte system have been studied with the electromotive force (EMF) measurement of the cell: Cs ion-selective electrode (ISE)vertical bar CsCl (m(A)), MgCl2 (m(B)), H2O vertical bar Ag/AgCl at 298.15 K and over total ionic strengths front (0.01 to 1.50) mol.kg(-1) for different ionic strength fractions y(B) of MgCl2 with y(B) = 0, 0.25, 0.50, and 0.75. 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 Nernstian response. The experimental results obey the Harned rule, and the Pitzer model can be used to describe this aqueous system satisfactorily. The osmotic coefficients, excess Gibbs energies, and activities of water of the mixtures were also calculated.