Values of the second virial coefficient B of HCl have been obtained from measurements of the excess molar enthalpy H-m(E) of {yAr + (1 - y)HCl} measured over the temperature range 210 K to 370 K at pressures close to atmospheric. A flow calorimetric apparatus of new design was used for the measurements. The value of B for HCl was extracted from the H-m(E) measurements using the spherical-core polar fluid potential of Storvick and Spurling. A knowledge of the second virial coefficient of argon and of an appropriate combining rule for the calculation of cross-terms was assumed. The values of B for HCl extracted from the measurements are slightly smaller than those obtained by Schramm and Leuchs using a mercury-free (p, V-m, T) technique.