In recent years, there have a number of studies using EMF method to determine the mean activity coefficients of alkali metal halides in electrolyte aqueous solution or aqueous-organic mixtures.For 1-1 type electrolyte, for example, KCl, the Pitzer equations were usually used for the mean activity coefficient (y±).Using the galvanic cell consisting of K-ISE and Ag-AgCl electrodes, we have studied the mean activity coefficients of KCl in the KC1 + glycol + H2O temary system at 298.15K.The y± decreases with the increasing of glycol content in the solvent mixture (Figure 1).Meanwhile the standard free energy of transference△ G0t is well estimated (Figure 2).