Vapour pressures of H2O, HNO3, HCl and HBr over supercooled aqueous mixtures with sulphuric acid have been calculated using an activity coefficient model, for 185 K<T<235 K, 02SO4)+wt%(HNO3) <70, and assuming HCl and HBr to be minor constituents. Predicted vapour pressures agree well with most laboratory data, and give confidence in the validity of the model. The results are parameterized as simple formulae, which reproduce the model results to within 40% and cover the entire stratospherically relevant range of composition and temperature. ¿ American Geophysical Union 1995