Abstract: In this paper the electromotive force of following cell: glass electrode |HCl (m)|Cl-ISE has been measured at molalities of HCl solutions from 0.005 to 15 mol kg~(-1) at 25 ℃. We found that Cl~--ISE Nernst's equation isn't followed. The mean activity, coefficients of HCl, calculated according to Nernsts equation are less than 1 in all molality range. On the basis of these facts, chemical adsorption of Cl-ion has been discussed. We belive that in concentrated hydrochloric acid Cl-ion are adsorbed on the surface of Cl-ISE, and form Ag~+-Cl~- and Ag~+-Cl_2~(2-). Potentails of Cl-ISE are determined by following equation: ε=ε°-Slog [γ_±m/(1+K_(γ_±)m)], (K=0.219), The activity coefficients of HCl which, calculated by this equation, are agree with that reported in literature.