The thermodn. properties of the HCl-NaCl-Me3COH-H2O system was studied by emf. measurements in the cells without liquid junction: Pt, H2(1.013 ?105 Pa)|HCl(m), Me3COH (x), H2O (1-x)|AgCl-Ag (A); Pt, H2(1.013 ?105 Pa)|HCl (mA), NaCl (mB), Me3COH (x), H2O (1-x)|AgCl-Ag (B) at constant total ionic strength I = 0.5, 1.0, 2.0 and 3.0 mol kg-1, from 5 to 45? where mA and mB are the molalities of HCl and NaCl, resp., x is the wt. percentage of tert-Bu alcohol in the mixed solvent, which is 10% during all measurement. The standard electrode potential of Ag-AgCl in the mixed solvent were determine from cell (A). The activity coefficients of HCl, gA in the mixed solvent system were determine from cell (B). The activity coefficients of HCl in HCl-NaCl solns. still obeys Harned rule. The standard transfer Gibbs free energies of HCl were calculated The primary, secondary and total medium effect of HCl were calculated and discussed.

Key words: WATER, METHYL GROUP, HYDROCHLORIC ACID, ETHANOL P, ELECTROLYTE, SODIUM CHLORIDE, ELECTRODE POTENTIAL, ACTIVITY COEFFICIENT, MULTICOMPONENT SYSTEM