Electromotive force (emf) measurements have been made using a Harned cell without a liquid junction of the type Pt, H-2(0.1013 25 MPa)/HCl(I-A), MeCl(I-B)/AgCl,Ag containing mixtures of(i) HCl and NaCl, (ii) HCl and LiCl, and (iii) HCl and BaCl2, at constant total ionic strengths of(4, 5, 6, and 7) mol kg(-1) and at a temperature of 298.15 K. From these emf measurements, the mean activity coefficients of hydrochloric acid in ternary solutions of HCl + NaCl + H2O, HCl + LiCl + H2O, and HCl + BaCl2 + H2O have been determined, using the Nernst equation. The Harned equation has been used to fit the mean activity coefficients of hydrochloric acid by a least squares method, and the values of the logarithm of the mean activity coefficient of pure hydrochloric acid, log gamma(A)(0), and the Harned interaction coefficients, alpha(AB) and beta(AB), together with their standard deviations are obtained. The results show that the measured mean activity coefficients of hydrochloric acid in the ternary solutions studied obey Harned’s rule at constant total ionic strengths from (4 to 7) mol kg(-1) and at a temperature of 298.15 K.