Electromotive force have been measured using a Earned cell without a liquid junction of the type Pt,H-2-(0.101 325MPa)/HCl(I-A),MeCl(I-B),KCl(I-C)/AgCl,Ag with Me = Ba, Li, and Na, respectively, at constant total ionic strengths of(4, 5, 6, and 7) mol kg(-1) and at a temperature of 298.15 K. In all the measurements, at each constant total ionic strength, the ionic strength of MeCl (M = Ba, Li, and Na) is taken as 20% of the total ionic strength. From the emf measurements, the mean activity coefficients of hydrochloric acid in quarternary mixtures of HCl + BaCl2 + KCl $ H2O, HCl + LiCl + KCl + H2O, and HCl + NaCl + KCl + H2O have been determined by the Nernst equation. It is found that the logarithm of the mean activity coefficient of hydrochloric acid in quarternary mixtures Studied in this work varies linearly with the ionic strength of KCl, when the total ionic strength of the mixture and the ionic strength of MeCl (Me = Ba, Li, and Na) are constant. An equation is derived which can be used to adequately describe the mean activity coefficients of hydrochloric acid in quarternary mixtures of HCl + BaCl2 + KCl + H2O, HCl + LiCl + KCl + H2O, and HCl + NaCl + KCl + H2O at constant total ionic strengths up to 7 mol kg(-1) and at a temperature of 298.15 K.