A flow mixing calorimeter has been used to measure the excess molar enthalpy H-m(E) of gaseous (water + hydrogen chloride) at the mole fraction y = 0.5, at standard atmospheric pressure, and over the temperature range 383.15 K to 483.15 K. Information about the strength of the water-hydrogen chloride interaction was obtained by analysing the measurements using a quasi-chemical association model. The second virial coefficient B-11 of water was written B-11 = B-11(ns) - RTK11. The non-specific interaction B-11(ns) between water molecules was calculated from the Stockmayer potential with parameters appropriate to a water-nonpolar fluid interaction, and the specific (hydrogen bonding) forces were described by the association model in terms of an equilibrium constant K-11(298.15 K) = 0.36 MPa-1 and an enthalpy of formation of DeltaH(11) = -(16.2 +/- 2) kJ . mol(-1) for the water-water hydrogen bond. Similarly for hydrogen chloride K-22(298.15 K) = 0.026 and DeltaH(22) = -8.3 kJ . mol(-1). The second virial cross coefficient was written B-12 = B-12(ns) (RTK12)/2, and from the temperature dependence of ln K-12 the enthalpy of formation of the water-hydrogen chloride interaction was found to be DeltaH(12) = -(13.4 +/- 2)kJ . mol(-1), and K-12(298.15 K) = 0.42 MPa-1. Values of the second virial cross coefficient are fitted by the equation B-12/(cm(3) . mol(-1)) = 18 - 18922 . (K/T) . 9.8415 . exp{1216 . (K/T)}. (C) 2003 Elsevier Science Ltd. All rights reserved.