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Activities of HCl in the HClCuCl2, HClNiCl2 and HClFeCl3 aqueous solution systems, which cannot be determined by the e.m.f. method, were calculated at 298 K by applying the McKay-Perring method based on the measured water activities. The water activities of these solutions and those of HClNaCl solutions were determined by a transpiration method, and the b values involved in the corrected Zdanovskii equation were evaluated as a function of water activity. The calculated values of a(HCl) of the aqueous solution system of HClNaCl agreed well with those determined by the e.m.f. method at both dilute and concentrated HCl levels. The activities of HCl in the solution systems being studied were then calculated. The change in a(HCl) with an increase in the chloride concentration is attributable to the change in ionic strength of the solution, and thus the concentration of the chemical species present in the solution.