A flow mixing calorimeter has been used to measure the excess molar enthalpy HE of gaseous (water + hydrogen sulphide) at the mole fraction gamma = 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 sulphide) 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 + non-polar 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. The second virial cross coefficient was written B-12 = B-12(ns) - (RTK12)/2, and from the temperature dependence of In K-12 the enthalpy of formation of the (water + hydrogen sulphide) interaction was found to be DeltaH(12) = -(12.0 +/- 2) kJ . mol(-1), and K-12 (298.15 K) = 0. 1 MPa-1. Values of the second virial cross coefficient are fitted by the equation B-12/(cm(3) - mol-1) = 56 - 53, 100 - (K/T) 2.814 - exp{1180 . (K/T)}. (C) 2003 Elsevier Science Ltd. All rights reserved.