A flow mixing calorimeter has been used to measure the excess molar enthalpy H-m(E) of gaseous (water + sulphur dioxide) at the mole fraction y = 0.5, at standard atmospheric pressure, and over the temperature range (383.2 to 483.2 K). Information about the strength of the water-sulphur dioxide 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. The second virial cross coefficient was written B-12 = B-12(ns) -(RTK12)/2, and from the temperature dependence of lnK(12) the enthalpy of formation of the water-sulphur dioxide interaction was found to be DeltaH(12) = -(14.9 +/- 2)kJ (.) mol(-1), and K-12(298.15K) = 0.18 MPa-1. Values of the second virial cross coefficient are fitted by the equation B-12/(cm(3) - mol(-1)) 29.6 - 39290 - (KIT) 3.0973 - exp{1493 (.) (K/T)}.