The mean activity coefficients of KCI in the ternary system KC1-CuC12-H20 were experimentally determined at a temperature of 298.15 K by the EMF method (electromotive force measurement) via a battery cell without a liquid junction, K-ISEIKCI (m1), CuC12 (m2) IC1-ISE (ISE = ion selective electrode), in the range of total ionic strength from 0.01 to 1.00 mol"kg-1 at different ionic strength fractions of CuC12 with yb = 0.20, 0.40, 0.60, and 0.80. The results show that K-ISE and CI-ISE have a good Nernst response in this work, and the mean activity coefficients of KC1 in the KC1-CuC12-H20 mixture solutions were calculated using the Nernst equation. Mixing interaction parameters Mccu and 1/Ac Cu,C1 in Pitzer's equation were evaluated from the present measurements of the mean activity coefficients of KCI. In addition, the osmotic coefficients, water activity, and the excess Gibbs energy of this system were calculated by Pitzer's equations.