Emf measurements have been made in cells without liquid junction of the type Pt;H2(1 atm)|HX(m1),MX(m2)|AgCl;Ag, containing mixtures of (i) HCI and (CH3)4NCI and (ii) HCI and (C2H5)4NCI, in different proportions, but at constant total ionic strength (μ= 3, 2, 1, 0.5, 0.2, 0.1). Measurements have been carried out at 10 ΔC intervals, over the temperature ranges 5-35 ΔC. The Ag|AgCI electrodes used, prepared by the thermoeiectrolytic method, gave closely agreeing results. The values for the standard electrode potential E Δ and the interaction coefficient α12 for each constant total ionic strength mixture at all the four temperatures have been evaluated by the computerized least-squares method. Interpretation of the results has been made in the light of the recent work of Pitzer. © 1978, American Chemical Society. All rights reserved.