Activity coefficients of HCl in (hydrogen chloride + holmium chloride) (aq) have been calculated from the observed e.m.f.s using the Nernst equation. The temperatures ranged from (278.15 to 328.15) K at 5 K intervals and at constant total ionic strengths of (0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, and 1.5) mol center dot kg(-1). Electromotive-force measurements were made on the cell without liquid junction of the type: Pt vertical bar H-2 (g,p = 101.325 kPa)vertical bar HCl(m(A)), HoCl3(m(B))vertical bar AgCl (s),vertical bar Ag(s) The results of the activity coefficients of HCl for this mixed electrolyte mixture have been interpreted in terms of the simpler Harned's equations and the ion-interaction model of Pitzer. Results show that the quadratic term is sufficient for the full range of Y-B (the ionic strength fraction of the salt) to 0.9 at all the ionic strengths studied. The Pitzer's mixing parameters S-theta,S-Ho, and psi(H,H,Cl) (including higher order electrostatic effects) and theta(H,Ho,Cl) and psi(H,Ho,Cl) (excluding higher order electrostatic effects) have been determined. These values at T = 298.15 K are: S-theta H,S-H = 0.115, psi(H,Ho,Cl) = -.071; and theta(H,Ho) = -.663, psi(H,Ho,Cl) = 0.165. The parameters obtained in this study reproduce the activity coefficients of HCl in the mixtures within 0.015 over the entire range of ionic strengths and within 0.009 for I >= 0.05 mol center dot kg(-1) over the entire temperature range. (c) 2004 Elsevier Ltd. All rights reserved.